Happy to report that macOS Monterey has fixed this issue. I believe it has to do with the improved external monitor support.
I tried the EFI you have upload on the other post:same result, around 37K
This screenshot shows the last few GB5 Metal and OpenCL scores for my PowerColor Red Devil RX 580 card. These scores are for my iMac1 system running Catalina and Big Sur. Not any significant difference between the way the card works in both OS's. I've not installed Monterey yet, as I need to change my SMBIOS to install the newest OS and to say I am reluctant to do so would be an understatement.
View attachment 532766
Here is a link to the top Big Sur Metal score, which as you said is significantly better than your Monterey score - 3637319 Not just the combined score but a lot of the individual scores are much better in Big Sur.
The question then is what background processes are happening to use the GPU's processing while you use Monterey?
Or is it an issue with the AMD drivers for the Polaris 20 GPU's in Monterey?
Apple Thunderbolt Display ... "ghost plug"...I may be seeing the same thing.
I am having a similar issue. With Mojave metal score with my RX 590 is around 57000. With Monterey, 37000.
I am using an Apple Thunderbolt Display as my primary monitor. In order to get my computer to boot, I use this thing called a "ghost plug" which tricks the computer into thinking another monitor is plugged in because the Thunderbolt Display isn't detected right away when the bios boots up. I usually keep it plugged in all the time.
With Mojave, I get the same metal scores with and without the ghost plug. With Monterey, if the ghost plug is unplugged, I get 37,000 for the metal score. When I plug it in, I am back to the 57,000 score I get with Mojave. With Mojave it makes no difference whether it is plugged in or not. Not sure if the issue is with Geekbench, or something about Monterey performing better with two monitors.