From what I've gathered, I think that you're better off picking one of the following two directions, and going with it:
- Keep the stock exhaust, and lose the cats
- Keep the stock cats, and lost the stock exhaust
I have TPIS long tubes, my stock cats, and the Corsa Ti exhaust. It's pretty loud as it is (although it's not nearly what I would call "too loud" -- but everyone's tollerence is different). I asked here whether or not I should lose the cats, and I was admonished that my car would quickly soud like a NASCAR car.
But, I've also read where plenty of people have ditched their cats, but keep the stock exhaust (or at least the stock headers), and they don't find their cars to be too loud.
So, my advice would be to consider keep EITHER the stock exhaust OR the cats stock if you're going ditch one of them.
-Kirk