So far as battery voltage changing your throwing motor speed......no problem. You have that "magic" custom PCB this year....all you need to do is build a circuit that measures RPM and allows the stamp to get feedback on actual motor speed.
As battery voltage falls, the stamp can simply use this feedback loop to pump more voltage to the motor to keep the speed constant.
But my big issue with the friction thrower is simple ball friction changes due to scuffing and dust/dirt as the matches move on.
I bet the friction will change as much as 50% as the day goes on.
