I use 200 cell 5 inch cats, similar to the Venom ones, they flow over 400kw. Not cheap though, around $200, and they don't have the flanges welded on yet.
The Venom 100 cell ones come with the flanges already welded in the correct place, and for a very reasonable price. I would buy one of them seeing you already have the cat back, they should flow even more, but I believe 100 cell isn't 'Legal' afaik.
Resonators are crap. Perhaps you can just get the muffler tucked up a bit more?