G'day RR,
nice bit of kit. The problem with putting in a 5 or 6 speed in our old bikes is that the motor never gets a rest. With a 4 speed it builds up to max then you change up the motor drops down into the torque band before increasing to max. Thus getting a little rest. With a 5 or 6'er it will nearly always be in the power band. I have witnessed quite reliable bikes with 4 speeds turn into hand grenades with 6 speeders fitted.
Cheers