thats the trouble with low compression pistons, unless your going to run huge amounts of boost theres not much point in having them, if you were to fit low comp pistons and run standard boost you would probably loose a small amount of power rather than gain any. you say the car was running 18 psi, well my rs has a chip, -31 actuator, mongoose system, grs intercooler, standard compression forged pistons and modified metering head but im on standard cam and turbo and it made just over 190 bhp at 18psi (peak). if he had standard compression pistons i reckon it would of been over 200bhp with his mods and boost level.
also as far as im aware that is at the wheels mate.