I just recently put on the gibson shorties on my 09 5.4 3v and i have a db performance (corsa cat back) exhaust on my truck. First off, biggest pain the *** I have done... lol like yotehunter said previously i didn't really notice it make the truck any louder but i can hear the air pass through or come out of the motor.
I should also mention the truck has an afe stg 2 cai and sct livewire x3 tune by Mike at 5 Star.
Can't say really much about mpg increase or anything bc my foot is quite heavy.
Hope this helps a little.

 

I just recently put on a set of Stainless Works long tube headers with high flow cats and x-pipe attached to 2 24" Magnapacks. I also have the K&N FIPK, BBK 80mm throttle body, and a 5 Star Tuned X3. I will say the headers have grossed the best performance gain out of all. I have not had my truck on the dyno, and am only going by seat of pants. the truck pulls pretty hard through out the rpm range, biggest gain from 2800 to 4500 rpm. By the way it's a 2010 F150 5.4 FX4 Screw.

 

I wish I had the money for a stainless works system. Oh well.

 

What kinda mpg do you get with all that?