It is a real pain, but there is only one RAW converter (that I know of) for the D810 and that is View NX2, which I have rarely ever used previous to today. So, these have been put through View NX2 as best as I could, but I would say they could be much better with my usual RAW converter, Capture One Pro 7, as I am used to it's operation and it's output. So, I cannot make any definitive IQ comparison between my D800E and the D810 due to this. I do think that ISO12,800 is better than the D800E, but this is just a seat of the pants judgement. ISO6400 and under seems about the same, but it is early days yet and I have only shot a few images.
Having tried a friends D4s, the D810's AF reminds me very much of it in the way it operates both in snappiness and accuracy to lock onto a target. I couldn't categorically say it was as good as the D4s as I don't have the opportunity to do a back to back comparo. Also, most of these shots were not all that taxing of the AF other than they were low light and it locked on brilliantly, but from my 45,000+ shots with the D800 and D800E, the D810 does seem a more confident AF system.
I shot today with my friend who has the Canon 5d MKIII and he now tells me he has mostly always shot in quiet mode. When he put it into normal mode, it is noisier than the D810 in normal mode. The D810 is quieter in quiet mode as well.
I haven't really used the D810 enough to make real judgement other than to say the AF does feel faster and seems more accurate, the VF seems a tad brighter, the shutter/mirror operation is so much quieter, it just seems to be a slightly more polished machine.
Most of these are at high ISO due to the low light at Taronga Zoo in Sydney. Most have been cropped and some quite a bit.
D810 + 300mm f2.8 VRII + 1.4x TCII, f5.6, 1/833sec, ISO12,800 handheld. Cropped about 20%
D810 + 300mm f2.8 VRII + 1.4x TCII, f4 (wide open), 1/500sec, ISO4,500 handheld. Cropped about 25%
D810 + 300mm f2.8 VRII + 1.4x TCII, f5, 1/500sec, ISO100 handheld. Cropped about 10%
D810 + 300mm f2.8 VRII + 1.4x TCII, f5, 1/500sec, ISO2,200 handheld. Cropped about 25%
D810 + 300mm f2.8 VRII + 1.4x TCII, f5, 1/500sec, ISO3,200 handheld.
D810 + 300mm f2.8 VRII, f7.1, 1/160sec, ISO3,200 handheld. Cropped about 30%
D810 + 300mm f2.8 VRII, f5.6, 1/250sec, ISO3,200 handheld. Cropped about 25%
D810 + 300mm f2.8 VRII + 2x TCIII, f5.6 (wide open), 1/250sec, ISO3,200 handheld.