If you couldn't double empower spells, this would be better than a '4.' But you can so it isn't.
Empower does the same job (in terms of damage) and is more flexible.
This feat is only better than Empower in very very limited situations:
A level 13+ wizard (a sorceror could just double empower and energy substitute the spell on the fly and would thereby either only face elemental resistance once (this spell has the potential to face it twice--a demon with fire and acid resist 20 for instance would likely resist 40 points of damage from an acid admixed fireball instead of 20) or could avoid it for the entire damage of the spell) who didn't know what foes he would be facing might use an admixed spell to be certain of having some damage type that would effect his opponents handy. Usually, however, this is not the case:
-Most creatures have multiple elemental resistances thus making this spell worse than a double empowered spell (see above)
-If one knows what one is facing, one could just energy substitute the whole spell and thereby avoid the elemental resistance entirely.
-If one doesn't know what one's facing, this is a bad feat for playing the odds. Substituting the whole spell to sonic damage would be better.