Believe it or not, even the Cat will make a sound difference. I'm not sure I'd run without it for that reason. I'm running a Magnaflow Cat, 24" resonator right behind it, and Magnaflow muffler. I'm finally pleased with the sound. I liked it before, but adding the resonator really did it for me.
The Resonator had to be modified to fit that tight spot, but it worked
I have pics of the install if you're interested. 3 of our members were there when it happened and got to witness the before and after.