In this situation, I dont think it really matters.

A deeper front wheel will always be a than a shallow front. The front breaks the wind first. An 808 front, 404 rear will be faster than a 404 front, 808 rear. But there are obvious handling issues if the winds are strong and your can't control it. On the other hand, if you can ride an 808 front, you can most definitely ride an 808, 1080, or disc rear. The rear end doesn't affect handling so you can go pretty deep without much impact on your handling. So the speed improvement of 404 -> 808 is far greater in the front than in the rear. I doubt you will notice much of an aerodynamic difference in the rear.