Are you driving the stepper motor with a stepper driver? For example the Pololu A4988 Pololu - A4988 Stepper Motor Driver Carrier which I have allows you to set the maximum current and then supply the motor with a much higher voltage to get it to move faster. The A4988 needs forced cooling for currents in excess of 1amp and the max is 2amps so I'm not sure if it is suitable for your motor. I presume other driver boards have similar features.
...R