The highest ISO I've used with the R5 II is 12,800. I took some shots of a black and white rabbit that had wandered into my yard one night, using only the light of a full moon. I used the RF 50mm f/1.2 wide open at 1/30th hand held, but my elbows were braced on a porch rail. The cameras high ISO NR was set to "standard" and the Raw file was processed in DPP. The image was quite usable, but I still ran the processed JPeG through Topaz DeNoise to post.
One thing that surprised me was that AF worked flawlessly in this scenario and locked quickly on to the rabbit, probably because it was black and white and provided good contrast, even under just moonlight. I've never tried this kind of shot with either of the R6 series or the R5, so I can't say if AF would have worked as well. I have shot in some pretty low light with the others using the RF 50mm f/1.2 and RF 15-35mm f/2.8 and AF worked just OK.
BTW, the posted link timed out, so I was unable to view the article. But I did Google the author and various key words and couldn't find anything but his blog on the R1 and 3.
Newton
