Since we recommend the HD806 as a family room projector, where its abundance of lumens is great for dealing with more than a little ambient light, it also stands to reason that we would favor a high contrast gray screen. The reason is simple: If you are in a room with ambient light, a high contrast gray surface will "reject" much of the side ambient light, further improving the watchability of a show or sporting event.
That same high contrast screen will lower black levels, rather painlessly. It's not a substitute for the Optoma's dynamic iris, but then, that iris is very noisy. The HC gray surface will also improve movie viewing enjoyment.
I've been watching the HD806 on my Stewart Firehawk G3 (high contrast gray), and today, enjoyed some pro football, with more ambient light than I'm usually able to handle with other projectors. The combination really worked well. I filled about 115 inches diagonal of my screen (largest image I can get where I have to place the projector), and the games looked really great. Had I used a typical white matte surface, the lighting levels in the room (mostly due to windows on one side wall), the projector would not have performed as well.
Only if you are going extremely large screen, with only ambient lighting back by the projector (where the HC gray surface wouldn't help), would I consider recommending a white surface screen.