The Limit on steppers is the average current which can flow in the winding(s).
This causes I x R losses which is heating. The torque of a stepper is set by the current in the windings. So the objective of a good stepper driver is to maintain the current and thus the torque. When you begin to run at higher speeds and the windings are turned on and off rapidly, the inductance of the stepper coil windings starts to diminish the average current which can flow. If higher voltage can be applied to the winding, then you can get back to the desired current and torque. As a practical matter, most stepper drivers use pulse width modulation along with a higher voltage, so that a FIXED power supply can be used, and the stepper driver modifies its pulse width in an effort to maintain the current level you set. You end up with a stepper and drive system which operates at a MUCH higher speed and the torque stays high, resulting in a good running system.
You need a fixed (and it can be unregulated) power supply at a higher voltage than the motor I x R sets, and a good chopper type stepper drive.
Sometimes they go to 20 x rated stepper voltage.