The first production car to ever receive a factory installed spoiler, was the 66 HEMI charger. The 66 spoiler is relatively small...but made a WORLD of difference at the track.

If memory serves, a vehicles aerodynamics start to have a noticeable effect at speed above 60mph. That would mean that the spoilers are JUST starting to be effective at about 60mph...so any speed over that, they should indeed be functioning.

That being said, the drag and weight associated with spoilers will indeed SLOW down a car a little...but you're trading speed for stability.