There are several things that can have an impact on the maximum speed of a stepper motor. As Jim mentioned, increasing your supply voltage with a properly adjusted current limit could help achieve higher step rates. Ramping the motor speed up from rest also helps reach higher speeds. Additionally, the external load on the stepper motor can cause the maximum speed to vary; the more torque your stepper motor needs to deliver, the lower its maximum step speed will be. This post by Ben has some tips for getting higher step rates that you might find helpful. Please note that although that post is dealing with a different stepper motor and driver, the information in it should still apply.
-Brandon