Having a shifting transmission is deffinatly a helpfull and worthwile modification; especially when you don't even have to worry about shifting. The only problem that i can see that might go wrong is that if you hit something at full speed and the wheel speed sensor gets knocked off or is rendered unable to sense wheel speed, you would be stuck in the gear that the robot was in before the sensor was disabled. Especially if you were in 4th gear. That battery might not last much longer!
