As far as I recall, the roof dam is all about drag and slowing the cars down.

The longitudinal fins on the rear window are part of the 'spin and lift abatement program'. I seem to remember they came into effect when all the cars started floating up when they got sideways to the wind, and that fin spoils the airflow and kills the lifting effect. The roof flaps that pop open when the roof sees the low pressure during a spin are there to create the same result.