[aerohead]

This is just some thinking out loud,but I am considering that the benefits are derived more in a crosswind.
The radius forms a sort of exoduct,much like a nice 1.5 radius ell found in plumbing and HVAC design.My Beechcraft Bonanza V-Tail has this at the engine bay extractors.
In a crosswind,as the air attacks from a vector off to the side,the wheel well trailing edge radius would provide a beneficial pressure gradient,forcing the airstream against the body,preventing a 'jet' to form as you may see during rain events,as the air erupts out the wheel well,in completely separated flow.
Since road vehicles typically see a crosswind,Honda may have designed for this.