im not the man on r31 autos, but i have an r33 auto(in a r33 gtst), and i blew my box( by that i mean i wore out the top gear clutches) because i was puttin more power through it only 160rwkw at the time, car had only did 80,000km
so when i got a new gearbox put in by MV autos he put in a stage 2 shift clutch basically replaced all the clutches with stronger ones, he said that the gearbox was now rated good enough for 230-240 rwkw
however other guys with autos haven't done a thing to there autos and they are fine, my car came with an trans cooler, but if your aim that high of an output i would suggest that as one of the first things to have