Not sure if you are using steppers or servos. I tested my Mesa 7i76e with a NEMA 23 stepper until it could go no faster.
My steppers maxed out at about 700 rpm and using 25x Microstepping, I could get about 90 kHz
I then backed off to 20x stepping. I then moved the step length back to 1500 and had no loss of performance but I never tested to see if I could increase the step rate. So from what you are saying, you are pushing the envelope to the max. The difference between 20x and 25x microsteps won't have any affect on machine accuracy as according to what I read, once you get past 16x microsteps, you don't get any appreciable change in accuracy.
