Do you really need 0.8 micron per step? That's the first thing I would change to a more sane value just as a mater of course. Reducing your microstepping can help you get more speed if you're running out of steprate between the motion controller and the motor drive, but with a decent motion controller and wiring I wouldn't expect only 28khz step rate to be a big problem for you even at your current settings. Lowering your acceleration can also help you reach higher top speeds as you start running out of torque at higher RPM, if you're losing steps on the motors.