Dunno, my opinion is that the "flatness" doesn't matter so much as does blanking off the air flow out of the wheel. Yes, it is fairly flat, but not flat-flat. And I mostly concentrated on insuring no flow could transfer. I did this by the obvious blanking panels, but also by running gasket around the circumference of the hubcap. And finally by using aluminum tape and closing off all the vents on the wheel (underneath the hub cap) itself.
These achieved a 2.0-2.5% gain in MPG.