There are 1000bhp GTRs in the world using standard boxes, they just dont do hard standing starts or rush the gear changes.

But with mega hard use people smash standard boxes with 500-550bhp.

R34 boxes are 6speed and not quite as strong as 32/33 5speeders, but not much difference.

Bloke whos doing my engine runs consistant 10.1s on a totally standard R32GTST-4 ?(ie almost same as GTR) transmission, and thats with 7500rpm foot to floor launches on wrinkle wall slicks with a 75bhp shot of gas too.
Breaks em a fair bit, but they last long enough for him to not bother going for an uprated one.