if the motor has already been rebuilt then i would say 10psi would wake the engine up considerably - you could possibly run more with careful tuning.
if you're trying to keep it on the cheap, ie a draw thru application, then i would keep 10psi your limit, with possibly ... and i hate to suggest it ... water injection. water / kero or water / metho would be a better bet, but still a bit archaic - certainly something EFI would fix.
but hey, they're strong little engines with a lot of scope for more power, so give it a shot. you learn more by experience in doing it yourself than you will reading about it huh?