[MrForbes]

You'd probably go for the point where you can control the speed, so it doesn't slow down noticeably. If you do some calculations as to the energy needed to shoot the ball, and the amount of time it takes, you can calculate how much power it takes to keep the motor at an almost constant speed. If the motor has enough power, then great...if not, it really doesn't matter as long as the shots are consistent, and you can get the wheel back up to speed before you want to shoot the next ball.