I haven't seen or read about any paint that could get more than 3 gain without hotspotting. Most of the solutions on this forum are about 1.3-1.5 gain I think. I will defer to MissMan--he'll have an opinion
such as Silver Fire, etc.
That said, a bigger issue is how to create the curved screen. To get the highest gain and ambient rejection, it would have to follow the curvature of the projection lens so that angular reflections focus back to one central location. This only happens in a retro-reflective screen (which you might want to consider such as a Dalite High Power 150" diagional for instance
). Any curving of stretched material will simply cause the center to flatten with some curvature near the edges leading to central hotspotting--I've tried that in the past.
I do remember back in the 3 tube crt days, that some people were using material stretched of a wood frame with a couple of vacuum blowers behind in a sealed chamber to suck air out and thus creating a curved screen. If you go this route, you'd need a pvc material or similar that is airtight. Actually good rubberized BOC might be enough air tight to work with an ordinary vacuum. I'm going out on a creative limb here, but you could make a frame using 1" x 6"s with a piece of plywood glued and sealed to the back. Drill a hole for a tube to suck air out. Then stretch and seal material to the front--paint it, etc.--, then using a vacuum suck air out the back and cap/seal the hole. Sounds easy, but getting it (and keeping it) airtight would probably be a nightmare. Anyway, my mind had fun thinking about it, though...
How did you get the curve in your existing screen?
I have a 144" flat screen with BOC sprayed with RS-MaxxMudd-LL using a Mits HC3800 in low lamp mode in a basement cave and it's great--bright enough and can handle overhead can light with no problem.