I don't think I was correct in making that statement--I've since noticed that the size and placement of air curtain ducts in production cars vary considerably. The Ford Ranger, for example, has a small duct well inboard of the wheel arch edge:
The Hyundai Veloster N uses a duct with an outside edge truncated well ahead of the wheel opening:
I'll be curious what you find. I've tried two designs of external duct now and found that one hurt stability and the other improved it.