Improved Metamagic affects the cost of applying metamagic feats individually, so your empowered maximized fireball will end up filling a 6th level spell slot.
From the "Player's Guide to Faerun":
Improved Metamagic (Su): At 10th level, an incantatrix has mastered metamagic to such an extent that whenever she uses a metamagic feat, the required increase in spell level (if any) is reduced by one (minimum +1 spell level). For example, a wizard incantatrix could prepare a quickened fireball as a 6th-level spell instead of a 7th-level spell.
Emphasis added by me.
This proves that it is how the individual feats costs, not the end result that is adjusted by this ability.
Follow up on comments
There is nothing on this in the Errata, but the description of Improved Metamagic goes on to say:
This benefit also applies to the incantatrix's other class abilities.
Thus, the DC for using her metamagic effect or cooperative metamagic
ability is reduced appropriately for the metamagic feats involved, and
she spends fewer charges when using metamagic spell trigger.
Again emphasis added by me.
I have always read this as modifying the feats (plural) cost individually, like we see with the Arcane Thesis feat or multiple Practical Metamagic feats.
The text can be interpreted to mean only a single-level reduction, or only with a single feat, but this doesn't seem to match with the implementation of similarly worded effects (Arcane Thesis, Practical Metamagic).
If this still isn't enough, then it would have to come down to a Game Master ruling.
