For their reporting on the VGs for their Lancer EVO, Mitsubishi mentioned that the TBL on the roof, just ahead of the backlight, was about 25mm thick.
If one takes this as a convenient working value, then it implies that the surface defects of your rear skirts are already fully embedded within the thickness of the boundary-layer. And the 'direction' of smearing indicates for an extremely 'healthy' transfer of kinetic energy ( momentum ) from the overlying streamline flow into the TBL.
As I said, I like it. You ought to be satisfied with your accomplishment.
