I have successfully applied this to our Crumble based Drag Racer.
With a 9 volt PP3 battery running the Crumble via a 7805 regulator, but with the Motor driver directly connected to the battery, we could get about 7.5 volts to the motors.
This is, of course, fully PWM controllable by the Crumble Pic processor, so we can steer down the line.
An extra 1500 microfarad capacitor across the crumble’s 5 volts did help matters.
About 45% faster!