I've never done a side-by-side comparison, which is what it would take to definitively answer your question.
My guess is not very much, if at all. First, these things, as rounded as they may be today, are still moving bricks. Second, having just gone through the yellow death (pollen season) here in the South...I can say with great authority that even "loose" pollen on the top of the hood center did not move much at all at highway speeds. This indicates to me that there is a sizable low pressue area in that part of the hood at speed - so the little scoop on the Sports would likely not create that much drag.
If anyone out there has any video of a Sport or other model w/scoop being tested in a wind tunnel, that might tell us. Anything else is going to be guesses and conjecture. As long as T4Rs have roof racks, stood-up noses with a large flat front bumper cover, and big honking side-view mirrors, and weigh 4000+ lbs, a little scoop on the hood is going to be the least of our MPG worries. IOW, these things already put off an aero wake only slightly smaller than a moving van - I wouldn't worry about mileage if you wanted a scoop.