Seymour XD is probably your next best option, but it's lower gain than the two above.
I'm debating Seymour XD vs. Svertson SAT-4K for mine. Planning 150" wide 16:9 screen with masking for CIA + IMAX. My last screen was a SeymourAV XD at 130" wide and 2.07:1 AR with CIA masking panels.
The Severtson SAT-4K, Screen Innovation's Gamma Maestro HD and Dragonfly/Snap AV whatever are the most inaccurate gain ratings for a white screens in the industry. It's because of them that we have to publish both unbenchmarked gain ratings (for calculations and actual science) and benchmarked gain ratings, where we roughly compare it with the basket of lying weasels out there.
Here is a picture of the SAT-4K next to the Stewart Studiotek 100. It's not color balanced, so don't judge the tones of either screen. This picture is just to show the obvious LOWER gain of the SAT-4K than the 1.0 gain Studiotek. In this rough comparison and controlled testing, I've found the ACTUAL gain of the SAT-4K to be 0.87, which is actually very good for a single-layer uncoated-thread woven screen. They didn't have to resort to the detail and contrast-blurring multiple layer trick, either. They should also know that gain isn't awesomeness and highlight that this screen reduces black levels by 13% and would offer a theoretical 25% reduction in room splash contamination resistance.
No woven screen can ever have that high a gain. For coated-thread woven screens we've maxed out the reflectivity in the Center Stage XD by increasing the retro-reflective powder in the pvc coating as high as it can go for both chemical and visual reasons, which is a patented formulation. If you must go higher gain than the XD, then the only options are coated films such as the microperfed Stewart materials.