The hi-power is retro reflective which means that the light is bounced back at the angle it is received. This is why the high power rejects ambient light so well (if the light isn't coming from near the users eyes, it won't be bounced as strongly towards the users eyes)
If you ceiling mount the projector, the 2.5 gain will be headed back to the ceiling. However the gain drop off is noticeable but not *HUGE*, you will still get a fantastic picture with a ceiling mounted projector, just not the full potential of the screen.
If you ARE going to ceiling mount, and want a high gain you could look at the Draper m2500 (although there are many reported problems with it).
Stewart of course also makes many fantastic screens for any application. (Thats what I'm saving up for)
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