Toby,
The Z1 won’t work for the setup you described, because its lens is too short. The smallest possible screen at a 15’ throw distance is over 120”-diagonal (16:9). So, if you really want the Z1, you will have to ceiling-mount it more upfront.
With the X1, the smallest screen size at 15’ is about 90”-diagonal in the 16:9 mode -- a nice size for the 12’-wide room.
As for the ambient light issue, you may want to consider what I did (our “living-room HT” doesn’t have a perfect light control, either): Use the retro-reflective Da-Lite High Power screen with the projector shelf-mounted at close to the eye level. My projector is sitting about 42” from the floor (at the lens-center) along the back wall, right above and behind the best seat. This configuration maximizes the screen gain (close to 3) and ambient-light rejection afforded by the High Power screen. With this setting, we can comfortably watch brighter materials such as TV programs in fully lit room at night.
