Ruben is correct I didn't leave any space between the in-wall other then an inch or so that is the frame width itself. I have the screen mounted directly on the bracket in front of my three 8" in-walls.
(Click to enlarge photos)
I have a professional acoustic engineer coming to calibrate the room and I should have some actual measurements when that happens but I'm putting up an additional 6 acoustic panels so that will be a few weeks away. I don't have the hard data to back it up but I can tell you that with myself and several other people in the room we have tested it to see if we could hear this "comb filtering" and none of us could. (I played regular SD, then HD then CD's all with out the screen and then all with the screen in place - I couldn't hear a difference at all nor could the others).
Ruben is correct about the close mounting though as far as getting a black out cloth when you mount that close you will likely see what ever is behind if it has any reflective qualities at all. In my case I had to remove the speaker grills as you could catch the metallic reflections and the speakers themselves are dark behind the grills so it is about 99% effective. Once in a while on the center speaker I can see a minor reflection on the tweeter but I have to have my head in the exact spot to see it and if I move even a 1/2 inch you can't see it again ( I may just put up a black out cloth in front of the tweeter itself rather the whole screen as it is just the one spot and only in a very specific position).
This is the closest photo I have of the screen in place and you can see it looks fine (I'd say perfect)
Over all I'm extremely pleased with the combination and I've had over 200 people through just the basement HT alone and not one person has made a comment that would tell me there are any sound or picture issues (and believe I was listening if there was an issue I'd be fixing it).
I think even if my acoustic engineer's data comes back showing comb filtering I still won't change my mounting if it doesn't bother me with out proof I can't see how it will with proof
I'll try to get a photo up showing the mounting better to show that it is flush with the wall (I don't have any on photo bucket right now to post and I'm not back home for a couple of days)
If you have any specific questions for me please let me know here or on my own thread and I'll be happy to answer them.