Whether you are running 6 psi or 20+ psi you are slipping if you are not running GripTec...
Belt slip isn't always noticeable. Often belt slip is noticed when the system squeals, squeaks or boost falls off. Did you know that naturally the belt can slip a very small percentage even on NA vehicles, one major factor to belts wearing. Belt wrap does play a key factor in minimizing belt movement but when it comes to driving a supercharger is hard to fully eliminate with just surface contact. Systems running 3.4 upper pulleys can see as much as a 1.5 psi gain when matching size for size pulleys.