My car boosts different amounts in different gears. I can boost 11psi in 1-3 but in 4th and 5th it'll only boost 9psi. The only two things that it could be is the boost controller or solenoid, or the wastegates. I have narrowed it down to those two. I blame it on the waste gates. My car sat for a long time in storage and I think my waste gates have rusted or there is dirt inside them. Since the amount of exhaust relative to the gear is greater in the lower gears, the exhaust can push harder on the waste gates. And so it would open up farther? I would say take them off and rebuild them or buy new ones. I'm going to try this out in the next few weeks. I have taken the solenoid apart and cleaned it and it functions correctly. Also check and make sure your solenoid is getting power. Hope I helped out a little even though I pretty much posted the same thing as you.
