most i've ever got so far on 93 octane with stock turbo/injectors and no knock is 19 psi. this was on a dry (but cold) 20 degree day. was using my turbotweak chip, but hadn't changed the timing in it.
otherwise, depending on the temp, i usually keep my boost at 17 psi. during really hot days of the summer, i've had to back it down to 15. i haven't played with my timing much when ambient is that hot, but might be able to get another lb of boost out of it. just need time to play around with it really