I agree the potential for improvement is probably tiny - likely too small to show up above the noise in coastdown testing.
In this case, I really do think a
subjective look via tuft testing might be the best way to make a decision.
There may be other reasons Honda may not have gone with a more convex shape - eg. maybe it would have been too likely to suffer curb damage. Just speculating. We know they didn't
completely optimize aerodynamics, so it's fair to be skeptical that a particular design choice is as good as it can be.
Tough to say for sure, other than this question gets pretty close to how much gild to put on the lilly.
FYI: I'm sure you've seen other Insights with smooth wheel covers, eg. member mattsmalls did DIY covers with fiberglass:
from:
http://ecomodder.com/forum/showthrea...tml#post173457
And I've seen moon type caps on other cars, eg Alfred State's competition (efficiency) Insight:
from:
http://ecomodder.com/forum/showthrea...l-20178-3.html