I had a spin tech. street xl installed a few weeks ago
Twin 2 1/4 inlet with a 3' out let useing about 12" of the orginal pipe then necked up to a 3 1/2 " polished tail pipe just behind the rear wheel.
Sound is much better, but It will never sound like a V 8.
Sounds sorta like a diesel on steroids.
I did get a small jump in MPG.