I have the Suzuki factory bug screen.  It came with the bike when I bought it, so I cannot comment on before and after fuel economy.

It does make a HUGE difference in rider comfort.  Instead of the air blasting me in the middle of the chest, it is deflected around me and over my helmet, making for a much more enjoyable ride, especially on the interstate.

With the carb mods, wind screen and factory exhaust, I am getting 55-60mpg.  I have been as low as 48mpg and as high as 64mpg.  It just depends on how hard I ride and how much stop and go I get caught in.

There are so many varibles affecting fuel mileage that it would take some very serious testing to verify what savings a windscreen makes.

If you're looking for a little relief from wind pressure, a windscreen is the way to go.  If you are looking for an improvement in fuel mileage there are other things that you can do that will yield much higher mileage increases -- reduce cruising speed, don't slow for corners, watch traffic lights far in advance so as to not stop, kill the engine and coast . . . .  There is a article in this month's Readers' Digest about people getting 100+ mpg in cars -- using practices that I feel are dangerous.

Crossing 150+ miles of the Mojave on CA-62 using Arizona gasoline, running 45-50 mph, I got over 70 mpg.  My normal city riding is around 45 mpg.  I am not very green.

Ah....   Checked some records and found:

My orginal May Mojave Moon Madness, in 2006, running the flyshield, had a 350 mile round trip average of 57.5 mpg.

A similar run in June '07 was testing the Street Shield / suitcase setup:

Round trip averaged 59 mpg -- about a 2.6% improvement.