So first of all, on the topic of Inspire Greatness:
Base, 2 THD
Song of the Heart, +1 THD
Focused Performance, x2 THD on one target
Words of Creation, x2 THD on one target
So you can either end up with +7 THD total, or +9 THD total, depending on whether you get the Song of the heart before or after the multipliers.
Also, on good thing to get:
Lilitu: gain a +2 profane bonus to saves by having her hug you once/day
A level of Fiend of Corruption: +3 untyped bonus to one of your stats
5 levels of Fiend of Possession: +4 profane bonus to all of your stats
Only problem with this is that there is the whole "do they lose the PrC abilities when their type changes" thing.