I prefer 3" with a race muffler and high flow cat......
But in reality try a magnaflow street series muffler, they tend to be a lot quieter than flowermaster, slp, or any other crap.
I saw flowmaster makes a new pipe called the Hushpower muffler. I would leave your current setup as it and install the long hushpower muffler inline after your cat (like a resonator)
Should make a big difference......