Nikon D800 Camera: exploring the limits of ISO


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:

I did a a little noise reduction only in Lightroom 4 (using Luminance=18, detail=94, contrast=60, and leaving the color noise reduction at default). I don't yet know what the limit is. I don't think I'll have that until I have more full-frame shots at high ISO, but right now, I'm thinking I can be close to matching the D700 high ISO performance, and that is far better than I would have thought.