My 97 F150 4.6 has a Gibson cat back SISO system on it. 3 inch pipe I think. Exit behind tire. It was on it when I bought it, so I don't know how much difference it made performance wise, but I like the sound. I wish it was a little louder when I get on it, but for long trips it's great. No drone, but a little growl when you get on it to remind you that it's there.
My 91 F250 7.5 has a muffler shop cat back system. 3 inch from the cat to a Vortex SIDO muffler, 2 1/2 pipes out the back with 2 1/2 SS straight tips.
Gained about 1MPG, but noticed no performance difference. Then again it's a 6100lb truck.....
Sounds good on cruise, not really loud. It has a real nice resonance at about 1500 rpm, but it actually gets a little quieter at WOT....kind of wierd.
It's lots of fun in parking garages (the few that I can get into) and tunnels.