I have used ClearPath motors for a router retrofit for a customer. I was happy with the result. ClearPath motors are designed as a direct replacement for open loop stepper motors, and close the loop internally. I would say at least an order of magnitude better than stepper motors. Easy to install and with the built in drives, takes a up lot less room in the electrical enclosure.
I'm not able to advise you of the size motors needed. But there are a number of writeups on Sieg X2D conversions, so if you use the same size motor as others have used in steppers, your performance will be better with the ClearPath motors in the same torque rating.