You don't need to regulate the power to the stepper motor/driver and feeding it a voltage higher than 5V should improve its speed a bit. The darlington array should be capable of 50V/500ma; the limitation of the stepper itself is heat.
Your controller board has a mysterious little jumper on it; if necessary, you can remove this jumper and add a resistor between those two pins to reduce the current through the motor and thus bring the heat of the motor under control. You'd want to add somewhere around 26 ohms for each volt over 5V you run the motor -- but again only if it seems like the motor is getting too hot.