The Mustang world uses the 17s I've read. We shall see if that's true.
Otherwise yes, compromise on top of compromise. But the bmw could use the rubber when it becomes a drag beast. So its kinda like recycling.
A world of options opens up once you notch the frame... that inch to the inside makes a huge difference. A 26" tall tire on a 17" wheel isn't the best choice for either type of racing (a 28" tire is worse, I've done it - do not recommend), but a 28" tall tire on an 18" wheel does improve the overall potential.