Well... 5 psi is what the wastegate does by itself and thats controlled by a spring. If you add a boost controller (or bleeder valve) you could have boost from 5 to whatever psi you want. What the controller does is fool the wastegate into thinking that its not getting a full 5 psi so it lets the boost increase. Alternatively you could change the wastegate spring.. but that would raise your minimum boost so its not a good idea. Make sure you add the extra fuel youll need to run higher boost..

-j0lt