There's nothing magical about the 17 and it's deflector. It's the same answers for any car.
It will help offset the loss of mpg with the sunroof open. Otherwise it will have little impact. However you need to be dong speeds this car is incapable of hitting for this number to be anything you are going to really notice at the pump.
About the only thing they do is cut down the noise and reduce some of the air coming into the cabin.
Where do you live that rocks hitting your roof are common? I gotta believe that is an extremely rare occurrence.