House cleaning:
Here are the numbers for why I decided the air dam didn't help.
I did AAA-BBB coastdown testing, using a phone to capture video of the speedometer so I could get a precise time measurement later at home on how quickly the speed dropped.
Methodology: I thoroughly warmed up the car with a ~25 minute highway drive. On the test road, ran it up to ~96-97 km/h (~55 mph), shifted to N at the same point each time, then coasted down to 40 km/h (~25 mph). The start point for timing was 95 km/h.
95-40 km/h coastdown runs, 3 with the air dam on, 3 with it off:
A1 (on): 75.93 s
A2 (on): 75.67 s
A3 (on): 77.67 s
A avg: 76.42 s
A stdev: 1.09
B1 (off): 78.07 s
B2 (off): 78.20 s
B3 (off): 78.43 s
B avg: 78.23 s
B stdev: 0.18
Difference: 1.81 s