The compression ratio and injectors will limit the NA when turbo'ing. With this kit, bigger innjectors and a larger fuel pump no reason you couldnt easily make 400+.
Don't forget that it's not all about BHP. Your clutch will need upgrading past 300 and your gearbox is being pushed to its limits after 400 (w58, auto will be less).
This well valued turbo kit I estimate on standard compression will give you somewhere around 300-320bhp with a good tune. Over 7psi boost your want to reduce compression ratio, it's just a common psi rating that Ive seen NA-T'ers reckon high compression will start to become very angry with boost