Depends on what type of R comp and under what use conditions, track or autox. Then comes track temperature and conditions. Why not just do a wider R comp?
It's really condition dependent, like rex said. Unless you have 9"+ wide wheels to mount the 255s to, your contact patch isn't going to be the full 255. I run 225 star specs when it's cold and/or wet, and 265 f/225 r r-comps when it's warm.