I made a diffuser this weekend. Results were noticeable on the freeway; much smoother ride. I suppose it was due to less turbulence. At some point, I'll get it powder coated.
http://i65.tinypic.com/ok95js.jpg
http://i66.tinypic.com/2dv0caa.jpg
After I took this photo, I added a washer to each end to make the panel lay flat. It looks much better now.