I have checked several rolled screws and found that the OD is definitely NOT concentric with the ball tracks. Some mfgs even state this in their specs. (IIRC)
I like the split bushing deal RICH, but I would still put it in a 4 jaw and indicate the ball races true .... just to be certain.
I also found that you can not expect the OD of the ballnut (even in a precision ground assy) to be exactly concentric.
The only true way I know of is to take the time (pita) to dial in the race ways.
Thanks,
Russ