Just posting a quick reply (I'll admit I didn't read every reply).
Unless you go with really (and I mean *REALLY*) expensive screens, and then you run into issues of matching a high performance projector to that screen to "compensate" for the acoustically "opaque" (not really transparent) screen. I would skip the idea personally. As for the M80s, let me tell you about my speaker choice path.
I started with M22 fronts and VP 100 center. When talking with the folks at Axiom, and we talked about cubic feet (volume of the listening area) I bumped up to M60 fronts and a VP150 center.
That was when I was really set up in a non-dedicated area. Now, I am in a 10' x 15' room and the M22s would certainly do the job, but I don't ever feel like my M60 fronts and VP150 are "overkill". Heck, the M60s would work wonderfully in your size room as well, but why not go for the M80s if you are looking at them anyway. They will be amazing for you.
