Increase the voltage, as long as you don't violate the insulation rateing of the motor windings current is significantly more important than voltage. Steppers, and motors in general use more current when stalled, they'll use less when in motion, the nameplate rateings you listed are stall rateings, which slow moving steppers run at, higher speed steppers can be run at any voltage that dosen't over current the motor or violate the insulation rating. 12 volts is NOT the insulation rateing of the motor. This is easiest to achieve using a constant current source for the stepper windings rather than a voltage source. Torque at higher speeds will be increased dramatically. It's hard to recommend a maximum supply voltage because I'm not very good with motors. One important variable that will help you chose the proper maximum voltage is the maximum power dissipation of the motor, if you're not sure and chose a voltage too high the motor will burn out, it's probably best to put some kind of temperature sensing element on the motor until you're sure you can maintain it at a safe temperature.