Ive been doing some custom races to test for potential rubber-banding. If its there, you don't see it in vs. mode. Obviously its harder to test it in outright GP, but in vs. mode I've tested tracks with both items and non-items. If a second-place bot gets knocked back to 4th without items, they don't place higher than that unless someone in front of them screws up. If they get items, however, they usually get strong stuff to brute force their way back.
This was done by checking replays on specific bots that came into matches high in point rankings. I didn't test the lower tier bots that usually finish towards the bottom, because in an average match, they have little shot at catching up anyway, and without items, it wasn't always easy to pull from 12th to 1st at the drop of a hat.
I'll keep testing, but in the meantime, the name's Lucavi31. Add me and get bodied online.