Whispbars are known to be pretty low profile and quiet (whisper?), but a lot of times, their really low clearance doesn't work for some boxes and other stuff you may throw on top. Having seen the OEM Mopar bars, I think they're the way to go. I believe they're produced by Thule, actually, so Thule would be the alternative if pricing was too high for the OEM. Just my $.02.
As for mileage, you won't receive much of a drop with just the bars. With the roof box on, your mileage will surely suffer. With my Audi wagon, full loaded with the roof box, I lost about 3-4mpg. With my Touareg V8, I probably only lost 1-2mpg, funny enough. It just sucked on gas anyway. I have to imagine the JGC I pick up will be the same 1-3mpg at most. A lot will depend on the weight inside the box, inside the vehicle, and your driving style.
KEEP IN MIND, products for the 14's may not be available yet...even if 11-13 equipment fits.
Pics for reference...