Alright guys, I found and fixed the problem. I don't know why F.I. put a mini cat in the pipe before the resonator, but the matrix inside the mini cat came loose and was spiraling all the way to the 18" resonator clogging the exhaust. The reason why I did not notice this is because I bought the catback used and the seller delivered it to me in one whole piece. He told me to install it without taking it apart to reduce the chance of ruining the paper gaskets. I took his words for it and this is the result after pulling the passenger side post cat pipe out.
It took us an hour to pull all the matrix out on the passenger side. I checked the driver side mini cat and it looked intact, but to be on the safe side I gutted it with some long drill bits and lots of cursing. Two hours later..
I used the OEM metal gaskets since the paper gaskets were trashed and so far no leak. The car is noticeably louder, especially now with those mini cats gutted. Power is there and noticeably faster than when I first bought it. I finally got to enjoy driving the Z again.