I had the same problem when I first got my GTR, drove like an absolute pig. Went to Tilbrooks and came up with about 150rwkw's too. Ended up being a boost leak on the pipe leaving the intercooler. They simply tightened it up and it pulled 207kw's on a car which only has pods, 3 inch cat and exhaust, and running 15PSI with stock ECU and just for the record ran 12.4 in that state.
About 3 years later lost nearly all power once on the freeway, thought a turbine wheel had dropped into the exhaust - ended up being a collapsed cat as per a few previous posts. Put a new exhaust system on and got 218kw's!
Have you tried someone else ECU??