gonna totally depend on your engine spec etc, esp stuff like compression, as if you build it for high comp you will have get massive gains as you can finally wang the boost up without det.

ive seen 80bhp on 2litre cars that i can remember seeing dyno sheets of, i cant remember exacts, but as so many skylines etc cant run more than 1.2bar on pump fuel but can run 2.2 on race as they specced so wrong for a road car, they can gain hundreds.

but if you built the engine right, with the right compression, so you can safely run the boost you WANT to on pump fuel, gains wont be as big.

still will be good gains tho, if you can afford the fuel all the time!