About 20 years ago I considered the Brightscreen for my Bronica ETRSi. I contacted Jim Beattie and asked about what effect of his screen on the metering accuracy. His reply (my words), "Try it and let me know". He sent the screen and I installed it in my ETRSi with AE Prism III. Viewfinder was noticeably brighter, and its brightness did not seem to be accompanied by loss of focus precision/repeatability. But the metering was indeed afffected.
And while the AE Prism III did have a metering compensation switch, it was not in the needed direction to compensate the Brightscreen (the switch only set + compensation, no - compensation). While I knew that I could simply dial in diferent ISO to compensate the unit, at the time I was shooting weddings and using different EI than the rated ISO film speeds, and I was putting different films into different backs, so I had to deal with 3 different [EI values:ISO values]...to much to keep straight in the heat of battle.
So I thanked Jim for the offer of the test, detailed my findings, and returned the Brightscreen to him.