rodzilla,
When I did my RF31 (same model as yours), I used 5/8" ball screws like most other people I have seen.The size of the screw has not given me any problems as far as I can tell. I have more problems with the operator (ME) than anything else! For the distances we are involved with and the speeds we typically run, I don't see us pushing the limits at all. There are more problems with the rigidity of the machine than the strength of the screws and that will limit speeds and loads more.
Bubba
Art
AKA Country Bubba (Older Than Dirt)