4 to 5 psi seems pretty low. I really had to stand on the throttle in a high gear to throw the code. I guess 30FF can be caused by alot of things based on the extensive time required to diagnose my car (severral hundred bucks at one dealer before i pulled the car and went to another). Some are relatively easy to fix (leak in the charge piping, vacuum leak) and some aren't.
I had a intermittent 30FF and constant rattle during warranty and only took the car in after the limp modes became more frequent (around 69.5k miles). They confirmed it wasn't a vacuum leak and replaced both turbos.
The service manager explained that "waste-gate play" was responsible for the rattle and the inability for the engine to make/maintain max-boost. I certainly don't baby my car, but she has always been bone-stock. They must have assembled mine on a Friday because I went through 4 fuel pumps in roughly 40k miles.
Best of luck with your car.
Just like Indiana...I love my whip