[Cd]

My guess is that you will get better gas mileage if you leave the spoiler at its factory installed position.
It grabs a bit of the air that is detached and gives it a clean break.
Personally, I would just add to the length of the spoiler to meet the template line .
It's my opinion that there is not going to be that much air flowing down the spoiler in the position you have it at. I would guess that the air will be too turbulent to do much good.

BTW, I think a large amount of the 'rubbernecking' with the front wheel covers is simply due to the color.
If you painted them matte black they would not stand out as much.
You have a really ingenious design going there. It seems people are making you feel ashamed of it, but I think the fake wheel idea will just amplify this effect.
Why not just paint the front and wheel covers matte black ?

At first glance it could simply look like a car without its hubcaps .