Thanks for asking!
To answer your second question first...
IMHO, if you can "test by driving", it's a reasonable option. As long as the driving conditions are consistent enough so you can really tell if the mod helped or hurt. Even so, I wouldn't claim it's as definitive a test as a good series of ABA tests under consistent conditions. But IMHO it's better than just assuming the mod is helping you.
In my case with my commute temporarily suspended, I did some testing of two versions of the project. First version was definitely a loser; coasts lost more speed with the spoiler on the car. Second version had 2 stiffeners added, possibly showed improvement. However the variability between runs was so large that I could see the test process itself wasn't very consistent (haven't posted those results to the
project thread).
I'm now on what I'm calling ver. #3, where I've considerably stiffened up the center horizontal panel. Four aluminum bar stiffeners. Haven't had time to do any testing and due to not working currently, I'm not doing the 55 mile drive to work either. Rainy today, nice tomorrow; but due to commitments at home I may not be able to get out for testing
.