Std pistons on 15psi aye i have heard of ppl using vauxhall turbo pistons and the limits of them are said to be 15psi with decompression plate how much does it lower it buy?

I never realy got how on zvhs make 220 bhp at 8psi which is great but then say when its running 20 odd psi itl make 250 bhp thats allot more boost to only produce an extra 30 bhp?