I think it works real nice. When I looked at the car, I first thought of lowering it. I checked out photos of people who dropped their stangs and thought w/ 17" rims it didn't look right. It is a large car and the wheel wells are certainly proportionate to it, but when dropped on 17's the space between the top of the tire to the wheel well is considerably smaller than the sides of the tire to the sides of the wheel well.
So then I started looking at 18's, but only for the back because the front 17's looked bigger than the rear 17's. Since the car is shaped like an arrow, where the back is bigger than the front, something had to be done to compensate for the optical illusion that made the rear wheels look smaller.
So, that's why the 18's on the back with 275 40 ZR18. I originally had 35 sidewalls when I bought the wheels and they didn't look right, far too low profile in comparison to stock 17 tires and they were still about the same total circumference as the 17's with 45 sidewalls. I had them swap the 35's out to the 40's and then it worked. It filled the rear wheel well better, like the 17's fit the front wells.
Also, the front stock rims are more flush with the side of the car, where the stock rear rims weren't. Even though the 18x9.5 made the rims stick out more in back, it still needed to come out at least a 1/4". So I bought 3/8" spacers and they are perfect! The front and rear wheels are equally flush with sides of the car now.
When you look at the car from all angles, most people don't realize I have 18's on the back because it looks proportionate all the way around. That's the kind of visual mods I like, the ones you don't notice, but somehow make the car look better against another Stang.
Hope that helps. Jeez, my toes are tired.