Hey. I'll chime in (as if on command)...
I'll make this quick as I am supposed to be watching a movie right now...
Sound absorption behind the speakers is a good thing.
A few inches is fine.
What screen material are you going to use?
Depending on the material, determines how close to the screen material you can place the speakers from the screen material itself. Many work best with at least 4 inches between the screen and the speaker, but I know of some good stuff that actually does OK with the speakers almost right up to the screen.
Let me know what screen material you are going to use, and we can figure that part out, and from there we can see how much room you have for space behind the speaker and the wall and how much space you have for sound absorption material. A curtain may not do as much as you think.
