Typically differentials are ran until they make horrendous noises or break. Most also have an average of 150,000-200,000 miles on them and are also over-engineered. F1 cars of twenty years ago ran a Hewland that was originally rated for 450 HP and they held up at 900 HP. If a small E30 is setup right it should be good for 300HP. But that's a wild ass guess.