Not to be an ass, but every time I see you start a thread, it's asking a question about what to do or what part to buy. Not sure if that's been pointed out yet.

In any case, I voted HKS. I've got one (D16 N/A then turbo) with the stock cat in place. It sounded great after it was broken in, deep and reasonably quiet without any ricer "bees in a can" noise.