A vetter style full fairing may be a net negative for ultimate performance, but its a pretty darn good piece, all things considered. If you are actually going to ride your motorcycle significant distances, losing 5mph off the top end (where you never go anyway) is a small price to pay for the comfort.
A naked bike is so dirty aerodynamically. Even if you increase the frontal area with a Vetter, you concurrently clean up the lines and the giveup is not as much as you would think.
I had a Vetter on an 836 engine for just a short while (stupid young kid, I sold it) and I don't remember much by way of negative in performance or gas mileage.
On one particularly high speed run to Phoenix from Tucson, I was 2 up with the Vetter and my buddy solo on a 750 Triumph was truly confounded.