I'm no expert, but if you're planning on running that much power through a standard turbo gearbox you're gonna want a much stronger box unless you NEVER plan on launching it.
The reason your mate probably never had a g/box problem before the paddle clutch is that the normal/uprated clutch was probably allowing a little slippage on take up, where as a proper paddle clutch grabs the flywheel much more efficiently therefore moving your theoretical weakpoint down the line. ie; to the box internals.
Quaife do a good uprated box for the rs turbo. A pal of mine had one on his mk4 zvh and that really did prove itself to be bomb proof. Hope you get it sorted, these conversions sound f**king awesome at full chat.
Short answer:
The paddle clutch wont break your gearbox, how you operate it might!