It funny - I literally am doing ISO tests for my review today and the D6 is included. When I downsample the a1 file to the same resolution as the D6 file, ISO performance at 6400 (the only one I've looked at so far) still favors the D6 - but not my much, maybe a half stop. It's basically like comparing a D850 / Z7 to the D6. Looking at comparative output, the D6 has a slight edge but nothing crazy. What really surprised me was that the color from the a1 was more vibrant at higher ISOs than any of the other cameras I tested (d6, Z7, a9ii, and R5). However, at the same time I find when the color is off, the a1 is tougher to wrangle in.
Keep an eye out, if all goes as planned I'm hoping to have the review out in the next week and a half.
Edit: Sorry, when I saw your post I had ISOs on the brain and thought that was what you were after
I'll also be talking about the AF etc but I'll tell you this much. Take the difference between the D6 and the D850 and multiply it by like 5 to get the difference between the D6 and the a1. To me, although there is a difference between the D850 and D6, it's not nearly the chasm of difference than between the a1 and the D6.