Ok well first the Servo drives are dealing with the encoders closing the loop at the drives and all your doing is sending it Step & Dir signals(pulses).

I did make a mistake when I said you'd get +68 M/min because I forgot about the ratio so RPM would be 625rpm at the pinion giving Max Velocity of 34.5 x 625 = 21.5M/min Rapids. . . BUT that would be with full 2000 rpm from motors which you won't get if 125Khz frequency. You'll only get 750(rpm) /3.2(ratio) = 234(rpm) x 34.5(pitch)=8085mm/min.
So really your wasting plenty of the motors potential and machine rapid speeds.

Why those figure didn't work I think can only be because your ratio is not what you think it is.?