I have a '05 SCab with the 5.4. I am planning on changing out the exhaust. My question is what is the difference (advantages/disadvantages) between SI/DO & running 2 mufflers. This is just be a catback system.

 

I have an '04 SCab 5.4 with a cat back dual exhaust and single Flowmaster Super 40 muffler. I've noticed no difference in performance or gas milage, but it was purchased just for the sound and I'm completely satisfied. Most cat-backs are sold as SI/DO. There may even be a space issue beneath the truck for a second muffler.