If you have the Panamorph the G11 on a 9' wide screen should be OK after calibration. My calculations give you around 20-22 ft-lamberts before calibration, after calibration and the Greyhawk that would give you a conservative 12-13 ft-lamberts. That is right around the SMPTE spec, how well that will hold up under ambient light is up to you. If it is light coming through your windows at night, you should be OK. 9' is probably the largest you would want to go, 8' would give you a little more headroom.
The Greyhawk will perform well in ambient light *if* you have enough brightness. It's really kind of hard to give you a recommendation, it depends on how much ambient and how much washout you can tolerate. You'd be best off asking for a sample for a projection test (you should be able to get Stewart to send you a good size piece). This is the best way to tell! If you find that the Greyhawk does not perform well in your ambient light, you could try a 1.3 gain or 1.5 gain. However, I would recommend stepping up to a G15 or getting dark curtains ( http://www.avsforum.com/ubb/smile.gif
), I prefer the darker blacks of a grey screen in ambient light. White screens really wash out.
In your case, I would not recommend a Da-Lite HC Da-mat because you are pushing the limits of the G11 a little and the higher gain of the Greyhawk will give you more wiggle-room along with the other benefits of a grey screen. At 8' the HC Da-mat might be possible.
Perhaps Blake from Da-Lite will come by and comment on the opacity of the HC Da-mat.
If you are going with a 8-9' motorized, I think you should probably get tab tensioning because your screen is large enough that it may form wrinkles. If you add tab-tensioning, Stewart screens are not really that much more expensive (and AVS has *really* good prices).