Remove the vacuum hose from the boost control actuator, plug the hose, and take the car for a short spin. Try one WOT and see if you have full boost. If you do, replace the actuator. Do not drive it like this longer than a short test run.
Now, observe the actuator arm coming out of the BCA. At idle it should be fully retracted. If you remove the vacuum hose with the engine running it should fully extend.
Remove the BCA and actuate the small lever by hand to see if it moves freely. If it binds, spray both ends of the pivot pin with WD40 or something like that.
2001 Bonneville SSEi (retired at 365,000 Kms.)
2014 Jeep Grand Cherokee Overland (yes, I know its not a GM)