We need to figure out some way to crack the formula for EXP scaling based on character level vs. enemy level, and then some equation to figure out whether Wisdom is a net gain or loss. Seems like the prevailing logic is that "Wisdom means more EXP, which means enemies give less EXP faster," despite the logic problem that if they're giving less EXP faster, it means you leveled faster, which means Wisdom is doing its primary function.
From what I remember of grinding Black Goblins in the Fjords, pre-Battle Tower, EXP scaling maxed out at 3 (or maybe it was 4) levels above you. When you're Lv10, killing a Lv14 enemy would have the same EXP multiplier on the gained EXP as you would get from a Lv20 enemy. Obviously the Lv20 would give more base EXP, but you'd get the same bonus percentage of EXP from each.
I have some old saves, maybe I'll go investigate.