how much gain would CTR pistons make in a stock b18a?

 

Probably not very much (outside of some serious detonation) without some kind of tuning. I'd do P30's instead.

 

I think the PCT's are kinda heavy. The .25OS slugs are anywhere from 330g to 335g each. A P72-A0 piston in std bore has a mass of about 302g to 303g. Forged pistons are usually sub-300g. The set of 81mm JUN Cosworth pistons I massed were about 290g for a similar +6cc dome to the PCT's.

You can find other, better ways of gaining mechanical compression than just by using a bigger dome. You just have to consider how much in the clearances you're losing to make sure not to go too far.