The winglets on the rear wheels, do they move with the alignment of the wheel? If you toe in at the rear, and if the entire aero assembly moves with the wheel alignment, then you are moving the leading edge closer to the diffuser. With toe out, you would be moving the trailing edge closer to the diffuser.
With toe out, you also create stronger jetting vortices from the tire. The wheel furniture, and the alignment of the wheel works together to strengthen the airflow traveling along the side of the diffuser. The closer the wheel winglet is to the car body, the more that area can function like a nozzle.