I'm set on a 2.40:1 screen because of the (somewhere between) 6 1/2 to 7 foot ceiling. This will give me the biggest picture possible in the available space. If I settled on a 1.78:1, then I would be loosing picture width. Also with all 3 speakers behind the screen, you need as wide a screen as possible.
Yes this would be a very good reason to go with a 2.40:1 screen and CIH. I purposely chose my house with a room where the ceiling my screen would hang on was about 10’ high so I could fit a large 1.78:1 screen with my M80s below it. Just so happened the widest screen was also the tallest screen I could fit.
Like I said I’m not the resident expert on 2.40:1 CIH but from what I’ve read I would put more money toward getting a better projector than buying a lens. However, those who have such setups may have a better opinion to share.