So you have a 50 trim with all boltons to support it but then you throw a stock measely 1.5" cat? I have a freeflow cat that I am willing to sell if you want it. Its 2.5 inches and quiets my exhaust quite a bit. I have a testpipe so I dont really need it any more.
Im not sure what your problem could be maybe turn up the boost some more
Check your timing too.. If your pulling 15 for vacuum your timing might be off or you have a leak somewhere.