Your alright upto 400bhp on the stock box. Start pushin more power out, the torque converter will eventually start slipping and give up.
That's one of my car's unfortunate weaknesses till I can get the boostlogic built box from America. I'm putting out 570bhp at the moment at 1.6 bar. So no wonder the auto box is dying on me slowly.
Don't know whether the TT box will fit on the na.
I'd recommend saving up for the boostlogic box. Spoke to the guys at boostlogic today and for the package upgrade, an auto box, a precision 3200 hi stall torque converter and shipping came to £2245! Heard its an awesome box and very quick on the strip and changes gear very quick. Worth looking into.