Your correct in your logic for the boost controller, but, not correct in your logic of the available boost.

If the vehicle, under steady state cruise conditions, requires 5 psi of boost, the system will be producing 5 psi. The boost controller will not have opened the wastegate and the system will be in equilibrium with the turbo having a constant output of 5 psi.

If you mash your foot down, the controller will prevent any wastegate action until precisely the setpoint........13.5 psi in your case........and then it will open the wastegate to hold the boost pressure at the setpoint.

With a standard wastegate, the gate starts to open earlier...........somewhere about 8-9 psi as a guess..........and has reached equilibrium at the "setpoint" of 13.5 psi.

The only benefit of the boost controller is the instantaneous action at the setpoint rather than the gradual action of the typical wastegate. It can offer higher boost in transient conditions..........but not under cruise.