sounds a lot like your cat is bad. to make sure, you could just jack your car up when its cold and beat on the cat with your hand, and if you hear a bunch of rattling, it's definitely your cat.
the best bolt on replacement is the random tech, which comes with flanges and an o2 bung hole. there is no other high flow for our car with this (all car sound cats need to be welded). random claims it will pass emissions, but i have yet to personally experience passing it with the cat (who knew illinois had emissions, i didnt
)