A125: You don't double it at all.
Incorporeal inhancement has a more recent printing in the Spell Compendium, which says each bonus "improves by the base amount for every 5 caster levels beyond 5th." Whichever order you apply them in, the bonus that has been modified by Fell Energy Spell is certainly not the "base amount".
Normal (20th level caster):
+4 AC, +4d8 bonus HP, +4 attack, +8 turn resistance
Fell Energy (20th level caster):
+6 AC, +4d8+2 bonus, +6 attack, +10 turn resistance
edit - Use it on desecrate for fun and profit.edit #2 - Actually, just be undead yourself and throw it on all of your buffs.
mage armor,
shield,
animalistic power,
protection from x,
create magic tattoo (just to name a few low-level ones. I'm not certain it'll work on the +1 CL use of
create magic tattoo since it doesn't use the magic word 'bonus'. CMT uses other than +1 CL and 10+CL/3 SR should work fine though.) Wouldn't work with
magic weapon, but probably would with
magic fang. I... think this is one of my new favorite Dragon Magazine feats.
edit #3 - use Fell Energy Spell with metamagic cost reduction on
wishes granting inherent ability score bonuses.
edit #4 - I forgot about
shroud of undeath! "You are treated as if you were
undead for the purpose of all spells and
effects that specifically affect undead creatures."