I assume you are seeing the 6 PSI boost at cruise or with no throttle? The turbo is so small on this engine it acts like a belt drive supercharger generating boost regardless of the throttle.
The crossover design and turbo manifold is from the same GM morons who designed the 4 PSI drop on the LLY Duramax engine. Airflow is something GM hasn't done well to the determent of MPG and power. GM's has gotten their ass kicked in the diesel market several times over it and they still haven't learned. Dodge was the first to drop a sewer pipe on for a exhaust.
So you not only have a turbo that is giving you issues for MPG, but, a manifold design that runs the driver side exhaust past 2 exhaust ports in a narrow channel. Turbulence adds to the narrow restriction there.
So even with a 4" exhaust and ATT turbo I am not getting 19 to 20 MPG. I do do better than the GM3 and it really depends on the load. The harder I work the truck by towing etc. the better the MPG improvement gets. More load the MPG goes down for both turbo's, but, it goes down more for the GM3 than the ATT. 7 MPG for the GM3 towing 10% grades and 10.4 MPG for the ATT same route.
Currently my 1993 is beating my Duramax in town by 1MPG.
With the above I say you have a few choices.
1) Go with a NA setup. Obtain 6.2 manifolds used cheap and go dual exhaust. Turn the fuel down with a N/A tune.
2) Go with a Banks manifold, ATT, and 4" exhaust.
3) Just get a ATT and 4" exhaust.