Martin - I may trim up the middle section a little bit, but not very much because it would only be a guess without more testing. Shot in the dark! And I don't have time to do more runs/number crunching before the weekend.
Quick calculation: at 50 km/h (30 mph), 1.8 additional seconds of coasting time equals 25 more meters (82 feet). Of course that won't be the
actual additional coasting distance from the air dam, since (a) the average speed of the 100-50 coast will obviously be higher than 50 km/h, and (b) the air dam's effectiveness decreases with speed. But I don't feel like trying to figure out the math. Math is hard.