An HC gray surface is very helpful in dealing with ambient light hitting the screen from the sides, so in a room like mind with windows (with blackout shades, but they leak some light) on one whole wall, the high contrast gray surface helps a great deal.
The gray surfaced screens may also be a good choice if you aren't going with a really large screen. After all, depending on whether you choose to use Brilliant Color, the BenQ in "best" mode, has somewhere between 866 and 1039 lumens, and technically, throwing 1000 lumens at, say a 100 inch white surfaced screen, is just too bright by many people's standards.
If you have chosen the BenQ especially because you are planning, or have a large screen (larger than 110", and really, 120 inch diagonal or larger), you are still going to have a bright image. To put it in perspective, let's use the Epson 6500UB and the JVC RS10 for comparison. The Epson measured about 470 lumens, just a bit more than half of the BenQ with its BC (Brilliant Color) off, and just less than half with it on. The JVC, one of the brighter projectors offers about 790 lumens so is just a tad less bright than the BenQ with its BC off. With BC on, the BenQ is almost 30% brighter.
Let's translate that into screen size. The image below gives you a pretty good idea of the room lighting I had for football viewing last weekend. In addition to the sunlight coming in from the bottom of one of the doors, four of the overhead recessed lights were on as well.
If you like BC on, on the BenQ W6000, then you can handle a screen about 18% larger diagonal, which translates to about 150 inches diagonal. Now that's a large screen!
So, it's a given, if movies are your thing, you can feed the BenQ a pretty large screen, regardless of whether you go high contrast gray, or, perhaps a slight gain white (1.3 - 1.4 gain), even better.
It's a slightly different story when it comes to viewing content like HDTV sports, and you don't want the room dark (cave-like), because, well, watching football with others is something you want to do with some decent light on in the room.
Again the W6000 has lots of lumens (1751 measured), but, in fairness, the color isn't so good in that best mode. Most of us will avoid that Dynamic/Native Lamp mode, unless the ambient light is out of control. Still the 1250 or so lumens in Standard mode, will do better than most others on whatever sized screen. I was rather pleased with the W6000 in that mode, on my Firehawk for this past weekend's college and NFL games. I had a moderate amount of ambient light in the room and the results were still pretty good (you can view the images in the HDTV section).
When considering your choices, remember that high contrast gray surfaces will have a narrower viewing cone than matte white surfaces, and that you will see some drop off of brightness, in the corners of the image. Still, such screens are very popular, because of the advantages they offer.
In my testing room, I was filling just about all of my 106 inch diagonal Carada Brilliant White screen, and when using the Standard mode for the combination of good brightness and good image quality, it really did produce a nice very bright image.
So, consider your room and desired screen size first. The BenQ has the lumens to work with a variety of screens, you just have to decide which of those works best in your room, for the type of content you like to watch.