I put a Flowmaster cat back on my 05 1500 HEMI quad 4x4, replaced the awful factory y pipe with a Magnaflow one and put on EBay Chinese shorty headers and got a nice performance boost. I never tested mileage but I didn't notice any obvious difference. These things cruise at 1700 RPM and probably only 25-30% throttle opening, I don't think you'll find much of a benefit mileage wise, BUT it's definitely worth the performance increase (0-60 in the mid-high 7's with mods, vs low to mid 8's stock), and I don't even have a tune yet. Honestly, if I had to do it again, I'd replace the factory muffler (the stock pipe is 3-3 1/2") and do headers and a y pipe. I read on a Ram forum someone dyno tested the y pipe mod and it alone was worth 8 hp (Did I mention the factory one is horrible?). The factory Y pipe on the new rams is even worse, I checked out a 2015 Ram 2500 Power Wagon and the one pipe going in is basically flat.