Biggest factor (to me anyway) is how long it takes the engine to drop revs and if you can get the MTX into the next highest gear before the revs drop all the way to idle. If I shift at anything below about 2500 rpms I'll have to blip the throttle a little to rev match engine speed with forward momentum. Shifting at 3000 or above lets me get into the next highest gear just as the revs are correct for a smooth clutch engagement. However I think it's better for the car and MTX if the shifts aren't done too quickly in order to keep the synchros happy and just match engine revs with the throttle before engaging the clutch.
Karl
