i've though about this problem, the original (non-mercedes-inspired) portion came into existence by asking myself if the size of the factory dam might have been determined by practical considerations such as curb clearence etc (the right side dam had scrup markings on the bottom when i got the car so they're likely as low as the average person would like them) also recents posts sugesting massive airdams could still work had me wondering is perhaps bigger was better.
so i extended the lower portion of the existing dam about 5 cm
you see the original here
my fe seemed to drop off the scale and the car wouldn't coast anymore... so after a few days i decited not to waste any more gass and cut them down a little to their curent form, this improved things although i can't compare to the base level
(truth is i kind of like the look so i didn't want to ditch them right away)
anyway, what i'm considdering now is to do away with the outer portion and go for something like on the tesla. i'll keep the merc dam in place
the tire dams will than deflect the air left and right of the tire, but the merc dams really feel ok for now, coasts and all feel pretty smooth.
this will have some air end up behind the merc-dams, since the merc-dams should create slightly less pressure behind them more air comming of the tires might go on the inside, so the wake on the outside of the tires will be smaller... the net ammount of air that goes into the wheelwells should be about the same because the dams force air that should normally go there to the center of the car (accelerated flow should be lower pressure thus the sort of downforce that makes a car feel a bit more stable)... anyway that's my uneducated theory