Stock is 15 and it's really pointless having more without an intercooler and extra fuel going in, in saying that when I first hooked up my boost gauge it showed a peak of 17 psi completely stock, I have a manual boost control valve fitted now to control the waste gate but have it set to relieve at 15 psi, I don't have a chip or intercooler.
Incidentally the peak comes at around 3200 - 3400 rpm, so even if you gave it more boost limit it'd be rare that you'd get to use it anyway, unless you drive it like a maniac all the time.