Guys, I wouldnt recommend running the S15 6 speed, if you are going to be running higher power than stock.
I owned an S15 Silvia SR20det Spec R for over 2 years. All i did to it was K&N air filter, minor increase in boost and a trick 3" mandrel bent exhaust system (with a seperate pipe from wastegate) this added 46kw at the wheels.
The car went well and i drove it quite hard through the gears and the odd skid and drift etc, but nothing really abusive.
The car had 38k kms on it when i bought it and was mint. After about 25,000 kms with the mod's the 6 speed was making very bad noises, it was stripped and inspected and found to be completly worn out, bearings and gear surfaces.
The gearbox shop said that they are known to be weak. Basically nissan squeased an extra gear into the same size case, so everything had to be smaller in size, hence the in-ability to handle the increase in torque.
Apparantly the Drift guys that run S15's out here, swap out the 6 speed in favour of the S14 5 speed for its strenght.