If all the other things are not done ie: swapping vacuum lines etc as per instruction other things won't work properly, as in it won't shut down smoothly etc, also it's good to disable the swirl butterfly as well as per the instructions, that keeps it fully open so maximum air flow is going through to the intake runners.
Depending on gear, revs and speed mine can be cruising at anywhere from 5 psi - 10 psi and when I accelerate pressure will build fairly quickly to about 16 psi, I do have a manual boost controller however, this stops the pressure from opening the waste gate partially too early and makes sure boost pressure is not wasted or leaked down the waste gate.