So I had a damn good tune from Torque Performance making 233.1kW ATW and also have a 2003 WRX STi.
Boost tuned to 22PSI peaking at 3,5000 RPM and tapering down to 18 from 6,000 RPM.
From 4,000 RPM AFR is tuned to 11.0 to keep EGT down and prevent knock. At 3,000 RPM AFR is 12.5.
Engine modifications are;
-3" Turbo Back (Catless)
-K&N Typhoon Kit (Intake)
-Grimspeed 3 Port Boost Solenoid
-GFB Mach 2
-Grimspeed Air Oil Seperator
My question is have you had a decent look at the car before taking it in?
For example looking for boost leaks, blow by, remove the restrictor pill?
Also given your boost response I'd think there may be an issue with your turbo.
Given that I had such a good tune from them I'd only assume you may have an issue with your car.