In short from what I have learned from my boy: a lot.
400+whp according to the boys overseas. New bearings, starting with a solid bottom end (ie: no broken rings, or damage to anything), and make sure you have a proper tune and it can handle a good deal of power. When I asked J he said that he doesn't recall anyone ever breaking a stock R's bottom end from over power. You best bet to find an accurate answer would be to look on the gtiroc or gtir.uk. The engine was built to rally race, it can handle a lot.