Ford Probe 1992 2,2 turbo

Stock boost of turbo is set at 6 psi, I changed it manually to 12 psi and car is working good. HP is increased, but it's still not enought for me. When I change boost for 13+ psi engine have problem to deliver fuel (don't know exacly). Is there someone who can tell me, what boost it can take without any more modifications? Because I don't know is my car working good, I read somewhere, one guy set turbo to 17 psi and it work.