I ran 3" on a 2.2 in my Daytona with a Mitsu at 18-20 psi with a big intercooler, didn't have any boost control problems, but I was running my own custom Turbonator LM 3-bar cal. It did wear out in a year and I went to the Garrett then.

I don't believe the turbo is causing the 4k flatline, I think it's the stock programming. Put a MBC on it and see what it does.