ive heard anything over 6psi, and your asking for trouble. yes tuning makes a huge difference but even then your reducing te engines reliability by quite a bit.
And before some guys on here with charged s52's come blasting about how theyve had their blown for years. Yes it also depends on how you drive it, if your a conservative driver that likes to play around once in a while, your ok. But if you are constantly beating on it, youll do some damage. Id recommend at least getting a commetic head gasket and arp head studs!
just my 2cents