I can only report on a 2007 HEMI improvement.
I added an AFE Stage 1 intake, Magnaflow SO/DO muffler and a DiabloSport Predator. My hwy mileage is up 4+ mpg. My around town is up almost 3 mpg.
I don't drive it easy, either. If I cruised at 65 vs. 75, I might get better mileage. If I wasn't always on the throttle around town, ditto.
But for some reason I just can't do that...