Like the other guys say m8!!!
Nothing lasts forever, when you shift grannystyle, hehe! I have also seen BPU cars kill there 6speed tranny while none of us APU drivers here in denmark have killed one yet!!!
Hardass-Clutches (hks tri) and Drag tyres are also a BIG killer of trannys!!!
I can safely say that if the power didn't get jambed the tranns will hold up just fine. Stay away from wheel hops. If you break the diff and it locks up as the gears come apart and the driveshaft had been upgraded the transmission will come apart. Even with a turbo like a T-78. Titan runs the Getrag on their race car and holds up just fine. The axles are a different story. Craig Paisley destroys gears on his g-force trannies and those things are rated for 2000 ft.lbs!! I'm sure the Getrag will not hold together for his race cars. I would say that if you plan on running this vehicle on the street 900 RWHP is the limit on street unless you were turn it down a little (very advisable). However, if you were racing it on strip the power is not as important as the way it is driven and the components you choose. Drag racing is very hard on synchros and replacement will only fix the problem you cause by hard shifting. Full throttle shifting helps out a lot if you can get your standalone to do so with speed and gear matching.
Titan ran the 6 spd for a WHILE. The unit in the racecar for the first season (up until vegas) was actually the same unit that came with the car, had Nero driving on 2835s for a few years ( I think it had about 40k miles on it). A big deal of it is whose driving, and the clutch. Throw a tilton in there and I don't see why 900-1000 whp would be a problem.