The wastegate actuator (gold thing under the turbo) sets the base boost pressure and the wastegate solenoid (little black electrical box on passenger front valvecover bolt) bleeds off pressure to raise the boost beyond that. The ECM has no feedback to control boost. It is just a guess that sets the duty cycle in the wastegate solenoid. the standard thing to do is get or make an adjustable wastegate where you can vary the length of the rod which also changes the base boost pressure. Do a few searches here and on
Turbo Regal Web Site for some nice primer basics on how it all works.
Greg Kring