Ideal boost pressure is relative to other supporting mods , which in turn effect the turbos efficiency, i.e the stock IC drops 4psi at the throttle body, most aftermarket ICs only drop 2 psi, where you see 18psi at the manifold, on the stock IC the turbo is pushing 22 psi, aftermaket IC would be 20psi.
Other factors to consider are intakes and downpipes, these reduce the load on the turbine / compressor wheels, once again, less load = greater efficiency, and as an added benefit a DP will also help reduce the EGT's, which in turn reduce the temp of the turbo housing allowing it to run cooler, less heatsoak of the charged air = greater efficiency.
Besides, tuning is about sooo much more than just boost pressure, a good tuner will achieve more power with less boost, than someone who will just turn up the wick and send you out the door.
Nearly all men can stand adversity. But if you want to test a man's character - give him power.
Abraham Lincoln