Ok, so the first results are in for the Jazz air dam.
I did a highway trip of about 350 miles, filled up twice and averaged about 50mpg. (Possibly 51mpg because I filled "too much" on the last fill-up.) For reference my lifetime average before that has been 44mpg. That being said I don't have enough data (and no ABA yet) to say anything definitive. Realistic guess about the improvement would be about 5-8%.
The Jazz is rated at 51mpg highway by EU standards. I guess I'll need one more mod to make the rated figures.
And I do pay a lot of attention to how I drive. I don't understand how car makers get away with their figures in EU. When it comes to adjusting my driving style, I don't see how I could improve mpg other than by slowing down. And I already drive pretty slow.