I'm re-reading the book "Forced Induction Performance Tuning". There's a chart on page 77 that's helpful. It shows what boost level is recommended for a modern stock engine (most of the book is focused on race engines) at different compression ratios. With a 10.2:1 CR and a nicely 80% efficient intercooler you're looking at maybe 5 psi as being safe. I think that helps to illustrate why our 11:1 engines with no intercooler would be pushing it with any kind of boost - whether from a turbo or nitrous.
An interesting website I found today is this one:
http://www.flat6innovations.com/performance.htm
It will be interesting to see what these guys can develop. It looks like some lower compression pistons and stronger connecting rods for the 3.2L may be on the horizon. It sure would be fun to build one of these engines for boost, but their prices make it a pretty costly proposition.
To really push it though you need an intercooler and they point out that there is a real packaging issue with fitting an intercooler on the Boxster. But with a good front trunk I would definitely be willing to sacrifice my rear trunk, it's close to the engine, and you could fit a very big intercooler back there. You'd have to modify the trunk to bring air in - maybe a scoop facing forward on the trunk. Then you'd have to vent air out the bottom of the trunk, but that's all do-able.
Seems like with a rebuilt bottom end for strength and low CR, plus an intercooler, twin turbos, and some exhaust work you could push 400 HP pretty reliably. Fun stuff to dream about! Throw in a wide body kit and wider rear wheels and you'll help that wheel spin problem.
Kirk