I have a Magnaflow cat back, Rugged Ridge snorkel, Superchips Traildash and switched to 91 octane gas. I gained 1.5 mpg. Thats my two cents.
Oh, and a K&N drop in

 

Some vehicles it is great, some it is not... On my Audi, sure, on my jeep...no thanks... The risk of messing up the computer seems considerably higher with my luck with Chrysler.

 

I added the Volant Power core CAI to my 2011 JK and I noticed it had more power to it. Now just need to pick an exhaust to help it out more....

 

An exhaust and a K&N intake on my 11' JKUR definately helped; absolutely no question. No; I don't have dyno results and I did the exhaust and the intake at the same time so I don't know how much which one made a difference but it definately improved the power of my 11'. It's easier to hold a certain mph on the highway w/o a downshift, and long uphill grades are definately a bit easier. The Jeep is still a slug but not quite as bad. I didn't keep track of any mpg's before or after.

 

I just picked up a K&N drop in filter for under $40. not much gain (1-2 HP) but I know I will never buy another air filter for my Jeep.