IMO, Your best bet for more speed would be to get a larger power supply(40v is the camtronics max right?) and increase the voltage to the motors, while keeping the amperage the same.
But no more than 20x the motors rated voltage per motor.
This would allow you to achieve faster RPMs and increase your rapids significantly.
But if youre really interested in more speed / torque ,then switch over to servos, and scrap the steppers. But it will be about 3 times or more,as much as a simular stepper setup.
Then again ,servos for a sherline is a bit overkill