Famous photographer Tony Northrup recently compared Sony a7RII (B&H/Adorama/Amazon) with new released Nikon D850 (Amazon/B&H/Adorama) on Dynamic Range and image quality. You can take a look at video above.
From the video, we can clearly see that already two years old Sony a7RII has similar image quality as new released Nikon D850 at high ISO tests.
ISO 1600: (Click image for large size)
ISO 25600: (Click image for large size)