Apparently these little flat tire spats/dams are quite sensitive to size & positioning. It's possible to increase Cd by going overboard, according to this article:
http://autospeed.com/cms/A_2456/article.html
I think the optimal route would be to add proper "3d" aerodynamic shapes ahead of the tires rather than spats/dams. Though of course that would be much more work to do.