425-450 is considered the safe limit. some have gone more, some less with failures. you need to have a great tune for more power. have a very experienced and reliable tuner tune the car.
Like bigcat said, it's all about the tune. What kills most engines is detonation, which is more likely to happen with higher boost. A good tuner will be able to eliminate detonation at higher boost levels. A car is only as reliable as its tune.
Dan
the tune is no problem, i have my car tuned at Modular Powerhouse (now MPH) so i know the tune will be safe and right on!! so its time to up the boost a little. . maybe 9.5 psi would be fun!! P.S. yes i tried the search function. . didn't find what i was lookin for really (yeah, maybe im inept at /search but hey.. im pretty not smart!!)