I hope everyone understands that deleting resos or the muffler or installing an aftermarket CBE will do little to nothing to increase power, since the stock CBE is so free flowing to begin with. There will be a little gain with deleting the second cat and replacing it with a race pipe. The big, I mean HUGE, power gain comes with an aftermarket DP/RP combo, especially if you have replaced the crappy factory airbox with a good CAI or SRI intake. Go catless with the DP if you're state is cool with it, or catted if not. The catted one gives almost as good gains. You will void your warranty, but if you want to play you have to be willing to pay.
You can change all the CBE components with or without resos or mufflers, with or without changes in diameter and all you're really changing is the tone and loudness of the exhaust. The stock CBE will free flow over 300 whp. Until you get above that level . . . its all a matter of what sound you want.
Just one opinion. Mod as you like, just know what you are getting and what you are not getting for your hard earned $$$.