I believe the issue is that the gearset for 6 speeds is fitted onto layshafts that are the same length as the 5 speed layshafts, so the gears themselves are narrower, have a lower mesh area and hence the rumour that they are "weaker" If you are going to be crashing gears, then this might be a problem, but personally I think the clutch would give out before the gearbox.
Also, from memory the S15 SR20DET ECU needs a signal from a tacho sensor built into their diff to function. There is a way to overcome this, but I'll have to look back at some of the magazine articles from the last 2 years lol. I think it was August 2005 in NZPC, but I can't find it right now...
I'll get back to you