Fairings work best on 451 or smaller front wheeled bikes as it is easier for the fairing to drop below the front axle level and sweep up to cover more of the rider's body.
On mine the fairing added a bit over 2# weight, had problems with the cutout for the frontwheel splitting at the top with failure of the fairing. As it is rather hilly in central Alabama, the benefit for me was not worth the weight. FWIW when I had the fairing on I noticed no difference in the wind resistance riding beside or behind DF riders. With the fairing off there is a distinct difference in the two position in perceived wind resistance. Finally I had to put car polish on the top of the fairing and leave it in the white, dried state as sunglare off the top of the fairing was really obnoxious. The polish made a relatively matte surface that stopped the glare and was easily replaced
Steve