In the grand scheme of things, this is a premium projector. While it is very well balanced, and has very good black levels, my first tendency is to recommend a high contrast gray screen for most people. I say this because it will overall lower black levels.
That said, this is a projector that in most cases will end up in a nice, light controlled dedicated theater room. If there are no ambient light issues that such a screen would help with, then, let’s keep the options open, and dig deeper.
If you go with a large screen (say 120 inches or a bit larger), the black levels (and bright levels) will be darker, of course, than on a smaller screen. As a result, larger screen folks may not feel the need to lower blacks further.
If your screen is masked you won’t notice the what would have been, the letterboxing, because of the dark surface of the mask. Even if no mask, on a larger screen, with a matte white surface, for example, the blacks should still look pretty good.
And there is a downside to high contrast gray screens. They will be a little dark in the corners, etc.
On the other hand, the high contrast gray screens may help if you aren’t in a fully light controlled room. The HD8600 is definitely one of the brighter projecrtors, and therefore the HD8600 is a good choice for a family room as well. The high contrast gray screens are good for “rejecting” side lighting.
Myself, I’m a black levels guy. I would, all, else, being equal, pick the high contrast gray solution, unless the room environment definitely pointed to another choice.