An appropriate current limit also ensures that your motor is not allowed to draw more current than it or your driver can handle. Running at a current limit higher than the motor is rated for risks damaging the motor.
As far as the maximum step rate you can achieve, the limitation will almost certainly be from your motor and not the driver. If you have a datasheet for your stepper motor, you might check to see if it has a pull-out torque curve to give you an idea of what the stepper motor is capable of; however, please note that there are a lot of factors specific to your setup that can affect the maximum achievable step rate. You can find some general tips for achieving higher step rates in this forum post by Ben.
Brandon