It's been a little while but I had two more thoughts on how to adjudicate empower turning. As it turns out, this feat alone can make an optimized cleric 6 / Radiant Servant of Pelor 1 (greater turns all day long) kill any and every undead creature they come across. You're talking about 9d6 (or more, with better items) damage, maximized (if that's how you do greater turns), x 1.5. So, 81 damage every round, with a save DC of 26+. At level 7. Throw even an 18 HD, nearly 200 HP monster against this cleric? Will save of ~13 STILL gives a 60% chance that the cleric will do over 1/3rd of its life every round. It's sick. (And again, the traditional cleric can't even dream of getting enough items/feats/whatever together to even think about affecting it.)
Empower turning isn't the only issue with destroy undead as-written: it's just the easiest way to completely abuse what is already an extremely over-powered ability. And, to be fair, there's no "official" interpretation of how this works: it's a traditional-turning feat that is being applied to a new mechanic, and I've been assuming it always does 1.5 x damage, since that's how its written. The simplest solution would probably be to grab the ban-hammer, but I like to avoid that at all costs. So, here are two more ways to run it:
a. Empower turning originally helped you affect more undead. It didn't kill them faster, although you could argue that affected undead were effectively "dealt" with. Still, a better way to think about it would be more undead fall under the power of the spell. So why not borrow a similar-sounding idea and just go with that instead:
Empower Turning is renamed Enlarge Turning. It increases your effective turning area for destroying undead to 60'.
Now why is that both good and not overpowered? Well, it doesn't multiply maxed damage into the stratosphere, so it's already an improvement. And it does follow the spirit of affecting more undead. But doubling the range sounds tremendous, right?
Well, except for the fact that I still believe the vast majority of all D&D battles are fought in an area that is too small for this to be effective. Or at least, at better way to say that might be: it will be effective about the right number of times for this feat to be useful, but not overpowered. And at the end of the day, it just helps you affect more undead.
b. Empower Spell is the feat this is obviously based on, and it's also very powerful - but then tempered by the idea that you have to use up a spell slot two levels higher in order to pull it off. Ok, so what if we applied that same kind of logic to turning? We could say that you are 2 effective cleric levels lower when you empower, but that's a bit messy: very low level clerics get zero use out of the ability, and very high level clerics really don't care. (What's another 2d6 x 1.5 damage when you're doing 15d6 x 1.5 already?) The save itself is also rather pointless to adjust (10% either way won't make enough difference). So what other "currency" do we have that we can force the cleric to burn? How about turns themselves?
Empower Turning: Multiply all variable, numeric effects of turning damage by 1.5. An empowered turn uses up two additional turning attempts (for a total of three turns). Note: You cannot empower a turn unless you have the additional turning attempts available.
So why is this better? Well, you're still going to be doing stratospheric damage, but the good thing is that you'll only get to do it about 3 times / day at low level, as opposed to 9 or 10. (Assuming Extra Turning was taken; otherwise only twice, or even once with low Cha.) And you're also burning through the thing that allows you to do damage, which means your "average" damage output is actually lower this way: you just get more of it at once, when you need it. And honestly, your "number of turns" doesn't really change much over time, so it's more of a fixed cost. Greater +Cha items help, and you can throw in a feat or item here or there, but it's not a guarenteed increase.
Now, you can still abuse the crap out of this: RSoP + Extra Turning a couple times + a few Nightsticks (Libris Mortis; +4 turns/day; a paltry 7500g) = you're pretty much going to be able to empower greater turns as many times as you'd need to, anyway. (And say you're down to just one turn left: hit yourself with a quick eagle's splendor and you've got all the bonus turns you need to get in one more empower - assuming you had no other enhancement bonuses to Cha already. Then again, this isn't really a new concern.) If this really bothers you, you could also add this caveat:
Special: Greater turns (the Sun domain granted power) can only be empowered using two additional greater turns.
The only downside to this caveat is that you might as well name it "NO FAT CHI- I MEAN, RSoP!" There aren't any other ways (that I am aware of, to date) that enable a character to get more than one greater turn per day than this one PrC. And if that's the case, it seems like that may be more of a house-rule situation for an individual GM to decide regarding the one broken PrC, as opposed to "let's make it a rule just incase people use that one thing."
Another way to adjust for this would be to minimize the normal impact and penalize the greater turn more heavily. For example, using three turns to do 1.5 x damage isn't particularly good from a "total damage done" sort of situation: you're giving up 3 x the damage for only 1.5. However, you're also getting 1.5 x the damage in one round, and actions are the game's most precious commodity.
A greater turn, however, is already doing 3 x the normal damage when empowered: it's x2 (roughly), and then x1.5 - which is actually x3, even though you might think it's just x2.5. That's worth giving up 3 turns EVERY time, since you're getting the exact same damage but quicker. The only thing holding you back is total turns. In this case, a better caveat might be:
Special: Greater turns (the Sun domain granted power) can be empowered using three additional turns.
Five total seems a bit over-the-top, even if it is more fair. Using this variant (of a variant), you might adjust the initial cost to just two turns instead of three. That still ramps up the power quite considerably, and anyone with Extra Turning is still going to have all the empowered turns that they need for an average day of encounters. But it makes the whole mechanic a bit more even when you take greater turns into consideration (double the damage = double the empower cost). And it still gives a much-needed nerf to the RSoP without singling it out quite as much.
So there are two more ideas for how to deal with this alternate class ability. For those keeping score at home, here's a complete recap of all my suggestions in this thread boiled down to the summary that I'm using in my current campaign's house rules (please note that rebuking has had less thought; I tried to cover it quickly and symmetrically, and I also realized that a saving throw for 1/2 HD or fewer was pointless):
Turning Undead
Replaced with: Destroy Undead (Su)
Destroying undead is a standard action that deals 1d6 points of damage per cleric level + the cleric's Cha bonus (if any) to all undead within 30’ of the cleric. The affected undead get a Will save (DC 10 + 1/2 cleric level + Cha modifier) for half damage. For example, a 6th level cleric with 16 Cha would do 6d6 + 3 damage and affected undead would have a DC 16 save. Undead with 1/2 or fewer hit dice than the turning cleric has levels do not get a saving throw against this effect.
Undead with turn resistance add 1 to their save against the effect and subtract 4 points of damage for each level of turn resistance they possess. For example, a vampire with +4 turn resistance adds 4 to his save and takes 16 fewer points of damage each time he takes turning damage.
When using this ability against incorporeal creatures, the cleric does not have to roll a 50% miss chance; turning hits them automatically.
The greater turning power of the Sun domain deals maximized damage. Any undead taken to zero HP by a greater turn are utterly destroyed.
Destroy undead still counts as turn undead for the purposes of prerequisites, use in feats, etc. Any ability that grants a bonus to turning checks instead grants a bonus to the save DC. Bonuses to turning damage apply to the damage dealt.
Rebuking Undead
Rebuking undead uses the same mechanic as destroy undead, but the rebuking cleric may choose to heal undead for 1d6 points per cleric level + Cha bonus up to their max HP (in which case no save is necessary and turn resistance does not apply) OR have all undead that fail their saves cower for one minute (an undead cleric may choose not to be affected by its own rebuke). Undead that have 1/2 or fewer hit dice than the rebuking cleric do not get a saving throw, and are commanded instead of cowering. The cleric may still only command a number of undead whose total HD does not exceed his level, and can give up command of existing undead to command new ones.
Additional/Changed Feats:
Empower Turning
Empower turning multiplies all variable, numeric effects of your turning damage by 1.5. An empowered turn uses up one additional turning attempt (for a total of two turns). An empowered greater turn (Sun domain granted power) uses up three additional turning attempts (for a total of four turns). Note: You cannot empower a turn unless you have the additional turning attempts available.
Enlarge Turning
Your effective turning area for destroying undead is increased to 60'.
Also, here are two optional monstrous feats:
Evade Turning
Your force of will allows you to shake off some positive energy damage.
Prereq: undead type, cannot be mindless, base Will save +5
Benefit: You take no damage from positive energy turns on a successful save.
Improved Evade Turning
You are extremely good at shaking off positive energy damage.
Prereq: undead type, Evade Turning, base Will save +8
Benefit: You only take 1/2 damage from positive energy turns on a failed save.
