I've gutted my cat totally ,its just a hollow shell now . 3" pipes ,no muffler . Could this hollow shell build up any back pressure to worrey about ? Also its very quite , not really as loud as what I thought it would be . Any comments ? The cat is 3" in and 3" out.

Cat will be more of a resonator now, and could be dampening the exhaust sound a little. Gutting it will certainly reduce resistance and back pressure as well.