This isn't why I drive the speed I do, but I do this exact same thing! I do mostly in-town driving so I haven't gotten my MPG that high yet (Currently 17.3) for an overall AVG, but I've taken some longer road trips (3.5+ hrs one-way) and been able to get about 21 MPG doing 75.
As for speed...
Totally worth it. I got a 5yr and with the paint correction, it cost me $950. It is so easy to keep clean. The water beading is amazing.
I use Chemical Guys products for my washes and cleaning and this thing always seems to look great.
Totally worth the investment in my opinion.
Cleaned her up, switched to black lug nuts, removed the plastic bumper, and installed a modular bumper.
I still need to get a front skid plate to match the modular bumper though.