Naw, the reason the AT's are faster in Supra's (or rather, more consistant) is because it allows the people to stand on the brake at the light and just let off and go...
Turbo's spooling and they dont have to slip the clutch (or feather it) or anything like that.

In any case.

If you want to modify your Automatic man, go for it; but I'd go wtih the juice and expect to repair the tranny at one point, and when that time comes just replace it with an MT.