Skaros said:
Actually, more than specific encounters that provide only anecdotal information, wouldn't further analysis of how multiply empowered spells compare against other spells that would normally fill the slot they are taking up provide stronger general evidence one way or the other?
For instance, if you can show that multiply empowering spells enough times to take up a lvl X spell slot tends to create spells that are clearly more powerful than normal lvl X spells, and that the extra power is great enough that having to spend a feat on "empower spell" doesn't justify it, then you have a pretty good case that allowing multiple empowers is unbalanced.
That being said, I'm not the expert to do this 
What do you think?
-Skaros
Okay, I can do that.
Let's take an Empowered Fireball and compare it to Cone of Cold. Both are 5th-Level Spells. Let's cast them both with a Level 15 Wizard.
Fireball [Empower]: 35 (3.5*10) + (3.5*10/2) = 52 damage
Cone of Cold: 52 damage (3.5*15)
Here they are equal power. Of course that doesn't stack Empower Spell. Now for a second example, which may be better because these two spells are actually more similar. Oh, and the fact that THIS one actually stacks Empower Spell, which is the point we're all addressing. We'll use a Twice-Empowered Fireball and a Delayed Blast Fireball. This time we'll use a Level 20 Wizard.
Fireball [Empower x2]: 35 (3.5*10) + (3.5*10/2)*2 = 70 damage
Delayed Blast Fireball: 70 damage (3.5*20)
Seems they're equal again.
Who says Anubis doesn't tell both sides of the story without consideration, eh? Of course, this doesn't show MUCH because there isn't a lot of stacking going on here, just two for the second example. Wait a sec, lemme give a third example with much more stacking going on. We'll use a Thrice-Empowered Flame Arrow and compare it to Meteor Swarm. A Level 20 Wizard will cast these spells
Flame Arrow [Empower x3]: 70 (3.5*4*5) + (3.5*4*5/2)*3 = 175
Meteor Swarm: 84 (3.5*24) + 21 (3.5*6) + 21 (3.5*6) + 21 (3.5*6) + 21 (3.5*6) = 168
(I admit I still don't quite know how Meteor Swarm is supposed to work, the thing in the PH is very very poor at describing it. I'm not sure how a single person could be in the direct path of four meteors that go in different directions, honestly.)
Now stacking Empower Spell is starting to show its effects. This does offer proof that it seems to be balanced at low levels, not getting unbalanced until high levels.
That said, perhaps the real problem is that many of you have not played an Epic game? In that case, the designers made a grave error not playtesting this out before releasing the ELH or allowing Empower Spell to stack . . .