No question the files from the D800 are great at low ISO's, but I haven't had too much chance to explore just high far I can go with pushing the low light limits. Photographing in overcast conditions with Auto ISO and a teleconverter, the lens combination was stopped down to 6.3. My shutter speed of 1000 resulted in an ISO of 2800. I have certainly produced good files on the D700 to ISO of 2000, and at times to 3200, but this camera with its increased pixel density?
Here is a major crop of a photo of a western tanager male done at those settings: