Just food for thought. When Cal tuned my car a couple years ago. My numbers hit a wall early (24psi) and didn't make much improvement as we increased the boost. My exhaust has got a bracket welded to my cross member, so I couldn't disconnect the exhaust without a hoist. I could only unbolt it and pry a screw driver in it to give me some more clearance. I had the same numbers you did with a GT4276 turbo (first GEN with a cast wheel). With only the screw driver opening I yielded another 55 rear wheel, proving my exhaust to be the culprit. I had a single 3" exhaust system on the car with a 5 to 4" downpipe. I later put on the RJC single 4" system and made 70 more rear wheel at the same boost level. You might want to look into improving your efficiency where your exhaust is concerned somewhere down the road.