I fail to see how closing the loop on a stepper-based system would be of any real value. The *only* way a stepper will lose position is if you attempt to drive it beyond its capability - either accelerating too fast, or exceeding it's torque capability. Once that's happened, how would you get it back to the correct position? The speed will be down, so you need to accelerate, which will exacerbate the problem. So, how are you going to "catch up"? With steppers, you must make sure you *never* exceed its capabilities. If you do that, then they never lose position. Your real problem is either your motors are too small, or you're driving them too hard.
Regards,
Ray L.