Flowmaster is good for sound, but that's about it. If you want less restriction, better fuel economy, and a little power, then I would suggest Dynamax, Magnaflow, and ******. You could probably be happy with only replacing mufflers. Obviously, headers, delete the cars (and be warned . . . a muffler shop cannot legally remove them, but if you remove them, they CAN install the exhaust system without cats), H-pipe (better for sound), and a pair of those mufflers I just mentioned would give you the best sound possible. I would YouTube some Tahoes/Yukons with those brands of mufflers to make sure you like them, whichever way you decide to build your system.
I still have my cats, and a pair of those restrictive Flowmaster mufflers. They will be gone once I take care of some other issues.