it can be done...but the 6 speed is a lot weaker than the 5 speed it replaced.
Most of my friends that have the 6 sped S15 swaped out the tranny as one of there first changes..the gears them selves are smaller and weaker than the ones in the 5 speed..
but the motor is kick ass
hey..if you got the mony HKS makes a sweet 6 speed for the SR20 in JDM land...