Shapeways materials do have a certain amount of tolerance. If your model measurements are outside the tolerance for the material contact Shapeways Customer Services, firstname.lastname@example.org, who'll be able to help you out.
The difference between WSF & polished WSF is the ceramic beads that do the polishing. The beads will only reach the places they'll reach and the places they reach end up so lovely & smooth, however that's probably not helpful to you just now.
FUD works to the nearest 0.033mm from my experience.