Squire James said:
My contention is that Empower Spell, being the only metamagic feat that can be efficiently "self-stacked" like this, is too powerful in itself to allow +50%, +100%, +150%, or more to spell damage. It mostly invalidates Twin Spell (that "mostly" in there because Twin Spell has some other uses like forcing 2 saving throws), and almost completely obliterates the Intensify Spell epic feat. All this for something a Wizard can take at 3rd level (and can start employing at 5th)?
First, empower spell is certainly not the only metamagic that can be efficiently self-stacked. Enlarge spell self stacks quite efficiently with spells like burning hands, cone of cold, or lightning bolt whose area of effect is dependant upon their range. Extend spell also stacks very efficiently with itself when used on long duration buff spells (For example, a 5x extended bull's strength cast by a 14th level sorceror would give 1d4+1 str for 70 hours. Similarly, the same sorceror can also cast a 2x extended Energy Buffer for a duration of 72 hours.
Empower Spell certainly sounds like it can be a problem when combined with the Incantatrix's Improved Metamagic. But that's a problem with Improved Metamagic and the Incantatrix not with Empower spell.
Regarding the other metamagic feats (Twin spell, Intensify spell, etc), there are other reasons to take them (Twin spell, for example, works on a wide variety of spells that Empower doesn't help--a Twinned slow, for instance would take out mass haste on round one and then slow the party's foes on round 2--it's more of a feat for use with "save or screw" or "save or die" spells than damage dealing ones). Intensify Spell is a product of a different mindset than Empower Spell much like Maximize Spell is. Both give statistically inferior but nonvariable results trading potential maximum effectiveness for guaranteed effectiveness.
Regarding the comparison of metamagic enhanced spells with other spells of the same (effective) level, there are two things to consider. First, the metamagicked spells
ought to be better in some situations. If they weren't, no one would ever use metamagic feats; they'd just prepare higher level spells in those slots. Second, metamagic enhanced spells have lower save DCs so there's a tradeoff. A 17th level wizard with a 26 int and greater spell focus: evocation could toss a double empowered cone of cold for 15d6x2 damage instead of a meteor swarm (24d6 damage in a small area--significantly less to most targets). Of course, the DC for the cone of cold would be 27 whereas the meteor swarm could effect one target without a save and would have a DC 31 save for the other targets. In many instances, the cone of cold will be better but there's still plenty of reason for the character to know meteor swarm (in this case, the reasons are called rogues, paladins, monks, and bards).