In the case of the Insight, I'm a Honda believer. I deeply suspect that they did the best that they could in the wind tunnel. It is true that the current wheel has some minor styling touches, and some small spoke slits, but it works very well with the current fender design, which incidently is far different than your "normal" car. If I had to venture a guess, and we are all guessing at this point, I'd guess that a FLAT cover would make a small, very small, improvement. If I test I'll do that first
The Moons stick out way too far.