I have probably the most severe issue with excess light output. My screen is only 80" wide. But, as Alan said, there is no such thing as too much light output. The goal is to use the excess light output to improve the image and an ND filter is the last resort. A very good one should not harm the image much, but it will not improve it.
The best thing to do is to use a low gain screen. I want to improve uniformity also, so I will use a matte screen. I talked to Stewart and they have suitable materials that I will test. I will probably use a 0.5 gain and the end result should be the same as using Stewart Snowmatte, but with half the gain. Compared to Snowmatte, the gray screen does have a slight shift toward blue. If the CMS works correctly, this is not a problem.
Regarding in room ANSI CR, the non-directionality of the matte screen does decrease it. The low gain goes in the opposite direction and helps with ANSI CR because anything coming back to the screen is decreased by the low gain. Compared to a 1.3 gain screen, a 0.5 gain screen will decrease the effect by 62%. Some of this will be lost because of the lack of directionality, the there should be a net gain. Also, the image will be much more uniform with a matte screen.