I think you're missing the point. There was never any mention of audience distance, only projector distance.
6'x9' would be an old 35mm slide format screen, which were 1:1.5 ratio. No video format is going to fit the full screen.
The widest lens I have seen work fairly well was a 0.8 lens which would give you a throw of about 7'3" for 9' wide. Even that lens had fall off at the edges, and was soft at the edges as well if it was in focus at the center,
Mac
I agree with all of the above, every short throw lens I have seen has been very soft in the edges and usually has a fair bit of barrel distortion too.
There is a new Panasonic lens out now however that actually performs rather well.
http://panasonic.net/avc/projector/products/dle030/This is the DLE030 - a 0.3:1 lens that works with the projector facing away from the screen right up close to it. When I saw it in the flesh the image quality was good and there was no need for keystone or other geometry correction to get a good image from it.
Light output appeared to be quite even across the screen too.
There is, of course, a trade-off. If I remember rightly, there is a 30% loss of light output through the lens. That means much brighter projectors for a given application, and the lens itself isn't the cheapest.
I only mentioned this as the OP is looking for a dual lamp high brightness projector, and the Panasonic range is well featured in this area. I would say on a 9ft wide screen you would be looking at either their 8 or 10K projectors with this lens!
-Gareth