I had a friend with a MY99 WRX,
TBE, CAI, injectors uncapped, fuel pump and aftermarket computer who installed nitrous and I'm 99% sure he took it to the drags at least 5 times "Yes he did have a few checks and some extra paperwork but it wasnt a problem"
Re: Stress on the car - yeah a kick of 50hp could damage your car - hell it could damage any car - its more about timing "when it is used" and a good tune.
Cant stress how important a good computer system/ tune is if your going to play with nitrous. My friends on the SAUWA forums have some horror stories of NOS in semi modded R33's