I can say I found the car lowering and air dam advice in that article, to be "spot-on" through my own experience at the Bonneville Salt Flats. My air dam is almost identical to the Hot Rod Camaro's (heavily influenced by that article, in fact), and after testing another design and no air dam at all, this design worked the best (at high speed). I had a few people tell me I was adding drag with that "snow-plow" bolted to the front of my car, so I tested it. Nope - it truly helps.
- Same thing with lowering the entire car. I had people tell me it would be better to raise the back end a bit - slope the hood & roof down, etc. So I tested that too. Nope - the least drag is with BOTH the front AND back ends as low as you can get them.
Now, this is for high speed and reducing lift, too. For minimum drag in a hypermiling type thing, extending the air dam all the way down to the ground is probably NOT the best - as discussed on here before. The idea is to make it low enough to "cover" the lowest hanging stuff under the car, but ONLY that low.