my car was in all sorts on tues / wed when it was up around 40 deg, i have stock turbo, cooler, ecu, and yeah she wasn't liking anymore then about 4-5psi, yeah she was shuddering, shaking, and stuttering, so i stopped boosting, but once the cold front hit, boy was boost nice, 12psi is fairly safe, would luv a cooler though, especially for summer