I like the 20s on the concept as well...
I think the Cobra should ride on 19" or 20". The wheel well is pretty large on the new stang, and the overall diameter of the rolling stock has grown as well. So the sidewall on said 19" or 20" will not be as short as the same on the previous generation of Mustang, and the ride quality is going to be much, much better due to the increased structural integrity and the front suspension with 'dual stage' bushings. 18s look good too. If 19s or 20s are used on the Cobra, please make the wheel as light as possible!