Okay first of all, running with no cat is just irresponsible. You're dumping raw exhaust fumes into the atmosphere and that's just no good.

If you are absolutely convinced that on an internally stock LS motor that the stock cat is holding you back, at least replace it with a high flow cat.

I'm not going to argue that the high flow cat doesn't have more flow capacity than the stocker. That said, an LS motor isn't going to push out enough exhaust gas such that the flow capacity of the stocker is exceeded.