Having destroyed two gearboxes while testing a maxxed out 24v C5, I can safely say the belt drive was also properly tested.
During all this abuse, the belt never slipped. Also, on a 64mile London-Brighton run last year using a 12v C5, the belt never slipped.
On this basis I would say if you have a gearbox in good condition, a new wider drive belt and a motor mod bracket fitted then your belt will not slip.
If it does slip, it has to be because something is not set up properly.
Make sure there is no play in the drive wheel on the end of the gearbox (bearings), that it's not worn with "hooked" teeth etc, make sure the motor and gearbox are sitting "on the wall" properly with the mod bracket a nice tight fit, check both rubber "bungs" are fitted between the chassis and the underneath of the seat,check you have the correct number of washers between the wheel and the large white drive plastic cog. Theres probably something else I've missed out, but you get the idea. It's quite a lot of fiddling around, but worth it in the end.