Re: the coasting.... Go faster.
Also, I've NEVER been able to get such stability in any testing! You're much better at this, or is it possible it was just a fluke?
I was able to measure a ~4% improvement in MPG in A-B-A testing of the much larger
Kammback/extension I made for my Firefly. Considering the one I made is significantly longer than your car's (and the starting Cd was significantly higher), I'd expect your results to be harder to spot. Looks to me like Mitsu did a good amount of the initial work with the factory spoiler.