The stock turbo is running at it's max efficiency at 25 pounds of boost. After that the stock turbo it losing its efficiency. Some people will tell you that they run their stock turbos to 29 pounds. They probably do, but the guys that designed the turbo (Garrett) say they are actually losing power after 25 pounds.
From the topics I have read on head gaskets, there is really no hard and fast number for stock gasket blowing boost pressures. I think the magic number I hear the most is anything above 40 pounds is borrowed time./ubbthreads/images/graemlins/shrug.gif.
I frequently see 32-34 pounds of boost pressure. No gasket problems (yet).
For what it's worth.