God evening. My roommate actually gave me the idea of trying this stuff. He's a roofer and they use this stuff on commercial buildings. It's a rubber membrane about 2mm thick and is extremely white and reflective. He snagged me a scrap piece yesterday and I finally got around to taking pictures. The image is substantially brighter than when using the blackout cloth. Below is an shot with the blackout cloth tacked over the membrane on the wall.

Here's a close up of the same area...

And here's an overall image with the membrane...

I need to get my hands on some da-lite samples and see the real difference (if any) there is.

As I mentioned you can find this stuff at commercial roofing projects and you might be able to sweet talk the foreman into giving you the scrap since a 10x12 piece really doesn't cover much of a commercial roof.

EDIT: The material is called TPO Single Ply Membrane (according to roofer roommate who snagged this piece for me.)

Ohhh looks cool, so is there any noticeable hotspotting? I would assume that there would be some for a more reflective screen, but not like the real high gain stuff where if you tilt your head, you're not in the sweetspot anymore...

As long as it's completely flat the brightness seems pretty even. I had a small wrinkle in the bottom left and it was overly bright but once I got it leveled out the lighting seems very uniform

There is no doubt it that blackout cloth does not have very high gain. If you look at the back of it you will see the image through it. That is all wasted light. Some recommend painting the back with a bright white paint. I would also suggest you consider painting the front of the BOC screen with a Pearl Clear Coat.

Now this roofing membrane is interesting. There are very white reflective roof paints available. I found, but have not tried, this Behr Roof paint. It is flat and they say it stays white even under the extreme conditions of being on the roof.

Another member over on Home Theater Shack has mentioned a vinyl roof paint called something like Snow Roof. I will try to get the exact name of it from him. He is a building contractor and has used the product for roofing purposes as well as to paint retractable screens.