I've been doing a bit of shooting with the D500 lately, and while doing some editing I came across this image of a Marsh Wren taken at extreme close quarters when it landed on a reed right behind me. The EXIF data shows that it was taken at ISO 2000, but there was very little noise in the file. I have to shoot a lot more before I will have a definitive conclusion, but I'm starting to think that with careful pushing of exposure to the right, great picture quality can be had up to and beyond ISO 2000 - maybe even up to 3200.

Here's a 100% crop of the image without any sharpening or noise reduction applied.