I see no problem with empower stacking with itself.
The incantrix is another matter altogether.
But, I decided to post because after reading this whole thread it seem slike everyone is mis-interpreting the empower feat.
I think it is very straightforward:
All variable, numeric effects of an empowered spell are increased by one-half. An empowered spell deals half again as much damage as normal, cures half again as many hit points, affects half again as many targets, etc., as appropriate. Saving throws and opposed rolls (such as the one the character makes when the character casts dispel magic) are not affected. Spells without random variables are not affected. An empowered spell uses up a spell slot two levels higher than the spell’s actual level.
Simply put it add 50% to any randomly determined spell effect.
The +1 added to rolls is included in the empower.
Those magic missles would be doing (1d4+1) * 1.5
Take for example, an empower cure light wounds cast by a sixth level cleric. It would heal 1d8+5 (it's +1 per level, +5 max) puls 50% more, or more plainly, (1d8+5) * 1.5. That is the only way the feat leave for interpretation "cures half again as many hit points..."
The empower does not make a continuous range of effect. For example the empowered CLW above would have a range of (9,10,12,13,15,16,18, 19).
An empowered fireball would be 10d6 * 1.5, causing a large but not continus range (15-90). This is different from 15d6 because the more dice you have more of a regression to the mean you create. The empower emphasizes the randomess and is a slave to it, in many ways empower is the opposite of maximize.
And speaking of maximize, a maximied empowered fireball would be the same as an empowered maximized fireball and it would do 60 + (50% of 10d6), not a flat 90.
g!