Rear discs are used routinely on outdoor tracks, where it can be windy, so I don't imagine it would be a problem on the road. However, most of the aerodynamic benefit comes from the front wheel, so IDK if you will notice a very significant improvement. What does make a big difference IME is using bladed / oval spokes instead of round spokes and to minimize the spoke count in the front and to a lesser extent in the rear. Finally, use the narrowest possible front rim and tire.