FrogReaver
The most respectful and polite poster ever
Only if it does the killing damage. But that's the best case for it. If we are assuming it is used in the first round to soften up a bunch of enemies and kill none, then it has no overkill.
But in reality you don't have control over whether it's used in the first round. Sometimes it will be, sometimes it won't be. Heck, sometimes you may even use an AOE on round 1 and round 2.
I don't think looking at efficiencies of single targeting damage splitting adds anything important to the discussion of AOE's. We can achieve the same results by looking at a case of N, 2N and 4N, 4.1N, 4.1N enemy Hps.
Or more precisely, the cases we are looking at are more likely to be fireball does N, party does 2.17N, enemies have 3.56N Hp. That we started out looking at an example where we needed precisely 100% of damage carryover efficiency to the next enemy is going to be a rare circumstance - so we shouldn't consider that the rule - and in the vast majority of cases, due to damage variability, to hit variability, multiple attacks, there is going to be a miniscule difference in actual impact between the 100% carryover damage efficiency case and the slightly below 100% carryover damage efficiency case.