I have a G11 and an ISCO with a 19 ft throw distance. I think if you spend the money on an ISCO you will get a larger picture (horizontal expansion) with good brightness. Remember if you use a G15 in a 16:9 setup without the lens, you are losing about 25% of the pixels-- 1365x1024 vs 1365x768--and therefore 25% of the brightness of the projector.
You would have good impact with the G15 on an 8 ft screen witha foot lambert calc of 31 (no ISCO), on an 11 ft screen it would be 22 (with ISCO).
I am running an 11 ft screen and am very pleased. With my G11, I have about 15 foot lamberts, more than enough, I think, based on what I have read. This is comparable to what Dean Mc Manis is running with his G15, ISCO and a 15 ft wide screen!
For me, the bright scenes are plenty bright, and i have good light control with a black rug and ceiling, so there is little washout.
Hope this helps