Again there isnt one set figure, it all depends on how you achieve it, on a nice effiecent GT series turbo, moderate boost and using ntirous you will be easily able to handle 500bhp on a standard bottom end, but with massive boost from a T4 spinning its guts out you wont.

Ignition timing is SO critical to the amount of strain on the pistons as well, but conversely of course the more you retard it the less power you get, so its a case of just being pointless to try as you are piling more and more boost in for less and less gains.