[cosmick]

I'm all in favor of scientific procedure, but not to the extent of undoing something as universally proven as an airdam. Too much work for no tangible reward. Then you have to reinstall it, even more work you could have just left alone. Plus the waste of time you could have spent on the undertray, enjoying even more MPG, so your removing it actually cost you even more $ than you thought.
You do want the tray sloping up from the rear axle up to the rear bumper.