Yes, AoEing lower level mobs is probably the most effective form of getting exp in this game (ala THM party in Wolf Camp in Nathalan). I won't argue with that. However, that still doesn't mean killing lower level mobs one at a time is less efficient than killing a mob with a 10 level difference.
I say this mostly because of how the dLv (the difference between the mob level and your level) works towards stat calculations in this game. I won't go into the details because it's long, but if you're curious look up Kaeko's posts. They are quite informative.
Found it:
Lodestone Link (under Targeting)Originally Posted by Final Fantasy XIV, The Lodestone