RB, You are extremely fortunate that you are blowthrough so you have the luxary of intercooling. Heat is the biggest precurser to the dreaded death rattle.
Datsonovic (Or a username very like that) says he has run 30psi on a stock engine, using only water injection to prevent piston destruction.
I hooked up a window washer pump to my boost gauge today, and was surprised to learn that it pumps 13psi. In the future when i begin to power tune my blown A15, I'll be implementing a water/alcohol injection set up for sure. This will allow much more spark advance than would otherwise be sustainable. i'd expect horsepower gains to go with it.