Light colored walls & ceilings are the bane of my home theater experience. With an Epson 8350, my contrast for worst-case scenarios (completely white scene w/ a tiny bit of black) went from 14:1 to 45:1 by getting a retroreflective (high contrast high power) screen & by using a black velvet curtain extending out 2ft from my screen on three sides (top, left, right). That's a 3x increase in contrast for worse-case scenarios (which, thankfully, does not make up most scenes in movies). Which is not at all trivial. Movies were unwatchable on my Elite 1.1 gain white screen.
In dark scenes w/ a little bit of white, I get ~1500:1 contrast w/ the HCHP screen, just for reference.
My point being, don't underestimate the need to reject ambient light, even if you're watching in the dark with all lights off. These projector/screen combos light up your room like anything, & the retroreflective screens at least serve to light up your room less (less scattering), and furthermore reject a lot of the light that *is* scattered.