I think that with good wheel skirts, the benefits of smooth wheel covers goes way down. The thing that might be better than smooth covers (under skirts) is to have sealed wheels? Anything moving that can accelerate air is going to add some drag. The Edison@ wheels are examples of this:
This is a different issue than air passing by the wheel opening. Once the wheel well is closed off (except where the wheel protrudes, naturally), it becomes essentially a fan/impeller churning the air in there; so having no openings or protrusions on the wheel is the next issue to solve, after wheel skirts are installed.