A few days ago I had a view of one at a stop light, and it looked like they were primarily directing air under the car; in part I was wondering whether that is actually the case, or just seemed to be because of my angle.
I think that item 6/7 in the honda parts picture above are called "Strakes". Those are $40 from Honda and $30 from the website. Pretty steep. I have been looking at dust pans as possible shapes for wheel air dams :
In the picture above, you would cut off the handle, which is "aiming" into the tire. The flat underpart of the dust pan would be the flat plane that pushes the air down.
But I haven't found the "perfect shape" of dustpan, if you know what I mean. Also, some other plastic product other than a dust pan should make a better shape, but I can't think of it.
I don't think the Insight's curve is the optimal aerodynamic shape. I think it's curved as a compromise to permit more ground clearance under the front of the car.
Check out some of the super-low Cd concepts that deflect air around the tires and you'll see either the usual blunt/hemispherical shape ahead of the wheel or a progressive taper, not the Insight's curve.
The totally flat tire spoilers appearing on more and more cars today also factor cost in to the compromise. (Flat is cheapest to design/make/replace.)
From what I have read, placement, size & shape of front wheel deflectors is reasonably important.
Properly designed front wheel deflectors can reduce Cd by about 0.01, according to SAE paper 2004-01-1307 (based on the specific vehicle tested - unnamed in the reference I saw):
However, [...] while these deflectors can reduce drag, if they are too large, drag can actually be increased. Further, in nearly all cases, the flat deflectors cause an increase in front lift.
They went on to show the results (with effect on lift & drag) of 4 or 5 variations, with changes in size, distance from the wheel, and width. Each had varying effects. Too large, too wide & too far forward actually increased both drag and lift.
I think a 3d shape would reduce the lift & forward placement problems right off the bat.
You'll note on most cars with the plastic slats ahead of the tires, they rarely extend out as far as the outside of the tread. This is to avoid deflecting airflow sideways excessively (increasing drag).