I've tried a few pix with my D800 at up to 25,000 and compared to the D700 at the same ISO, the D800 wins easily with no smear from the specular highlights that need lots of post. That said, I'm back from a month in Mallorca, Greece and Serbia shooting some stuff on the tripod and others using Hail Mary lighting and hand held.
My main body was the D800 with 24-120 f/4 and second the D700 with 17-35 f/2.8. Sometimes, I'd swap lenses when i knew I needed more res and a wider FOV.
I just looked at one of my Hail Mary photos shot in an 800-year old monastery with light streaming through the door and cranking up the D800 to just 6400. It was contrast city when I was shooting and I was blessed to be the fly on the wall. The image holds together well all the way from the sun-lit floor to the with aggressive NR in ACR (60 for luminance, 35 color), the image more than makes the grade!
For printing, I've got to reduce the size of the files which make the files even nicer.
In the overall scheme of things, once the ink hits the paper for me, I really can't tell the difference from one of my cameras when I'm printing smaller prints (less than 16x24), but looking at my edits from my time at this monastery, I almost wish I had a second D800 cranked up even a stop higher. I'm very sure that my images will work quite well at 20x30 and larger.
I tend to ignore the math and simply shoot the camera then deal with making an image. The math, though important to understanding the craft seldom comes across as an important element my final images. The final image is King in my book!
Attached is the final print (reduced) with a full-res crop of a section of the image from the same file.
As always, one's results may vary.
Bottom line for me is that the D800 makes the grade for high ISO shooting.