How much does a hood scoop actually help?

To get any ram or forced air effect, you might actually have to get up pretty high to get into a real air stream.

I've always read that you need to get up 4-6" (or more) into the air before you start to see any "Ram Air" effects. It had to do with surface friction or something and there being a blanket of stagnant air over the surface. If you think about what you see in wind-tunnel pictures, the smoke trail they run always sits up off the surface a bit (see pic).

mercedes-amg-gt-coupe-wind-tunnel-testing.jpg