There's no AP Plus. Only HP/MP/Magic/Speed/Luck/EXP/Gil Plus. The only way to increase AP gain is to use Double or Triple growth equipment.
I see. I must of confused it with EXP plus. (This goes to show that I haven't played the game in quite some time, that's why I'm forgetting some things.)
So it definetly would be a good thing to put in the game, I agree.