There are 2 different things at work here. For battery longevity, it doesn't matter. The car will not allow the batteries to be charged too quickly based on their current state of charge, temperature, etc. So you can charge at any rate you want (AC, not supercharging) and the batteries will be fine.
Tomsax's work that Lloyd is referencing is related to charging efficiency. This is where the maximum amount of power from the wall goes into the battery. Above or below this point a slightly higher percentage of incoming power is being used as overhead to manage and cool the batteries.