And now for some results.
Since I installed the air dam, nobody has had anything to say about the air dam, while several people commented on the topper. I take this to mean that the air dam looks close enough to "normal" that nobody notices it enough to mention.
I now have MPG results from December 2016 thru February 2017, the three months that I use to calculate winter gas mileage. My mileage for Winter 2016-17 was 33.0 MPG, while the best previous winter was 2015-16, where I got 31.75 MPG. I no longer drive three miles each way to work, so that should improve mileage by 0.7 to 1.0 MPG. The improvement attributable to the air dam is thus 0.25 to 0.55 MPG.
It will be interesting to see what I get for mileage next summer, because I averaged 38.6 MPG last summer. I really want to get to 40 MPG.
The vacuum gauge plus wheel covers helped increase summer 2015 mileage to 38.5 MPG, while summer 2016 mileage was 38.6 MPG without the wheel covers. Drove 10,588.6 miles in 2016 at 35.0 MPG.