So does this mean any projector with a Autozoom will fill it up and NOT have black bars?
For the record, you can do it manually, it's just something you'd have to adjust at startup. My point here is autozoom is not necessarily a requirement. It just makes it easier. And with lens memory, you set it once and forget it.
MOST projectors are native 16:9 (like my Panny4K), so there ARE black bars at 2:35. The trick is that by building a screen wall and making it all black, the bars are projected onto the black wall and become virtually invisible. The Panny projects black enough bars that it doesn't bother me when they are projected on the side of the screen. I guess I'm just not that picky or my eyes are bad enough that it doesn't bother me. Either way, the image fills the screen top to bottom and fills the entire screen (2:35) or has narrow bars on the side (16:9) and even wider side bars for standard TV (4:3).
You can search threads/forums for more information by looking for "CIH" or "Constant Image Height."
Although I haven't researched TONS of projectors, I think that if go to the Display forums and runs the calcs on whatever model you are looking at, you'll find that most projectors will work out great for you in the ranges (seating distance/screen size/projector distance) you are talking about. The trick is to try to find a projector that won't require max zoom in or out at your distances and picture quality should be fine.
Frankly, I'm not the one to answer about screen gain. I chose an acoustically transparent material that, if I recall, is in the 1.1 range and I'm quite happy with it in my light controlled room.YMMV.