Wow, I know I'm late to this conversation, but for the sake of posterity i thought I'd correct a couple mistakes here. The way you are looking at this stepper is like the way you'd analyse a dc motor. This is incorrect. While a stepper runs off dc current it is a form of ac motor. The resistance in the motor needs to be partially inductive (aka complex impedance). You need to use a chopper drive to limit current while increasing voltage. To properly use higher voltage you need to drive the circuit at higher frequencies. Each motor has a fixed distance it moves with each pulse. So to go faster you need to pulse quicker. Pulsing quicker leads to induce currents In the rotor and this reduces efficiency. To overcome this we increase voltage and decrease pulse length to generate more force. More force gives faster acceleration between steps and thus higher speeds.