Do you have the 2:1, or 3:1 reduction? And what speed are your rapids set to.
And what kind of servos?
At 1000 rpm, you're 960oz steppers have about 200oz of torque.
A 200w AC servo has about 250oz of peak torque, all the way to 3000rpm. If you went with a 10:1 gearbox, you'd have much more power with the 200w servo. If you want to keep the existing belt drive, then you'd probably want a bigger servo.