This I would disagree with. The D3s/Df/D4/D4s all equal/better the D8xx at high ISO even after downsampling/normalizing the D800 files. Even the D3/D700 can equal it in some aspects/areas. Most ISO tests on the web are not worth much in regards to using higher ISO's due to lower light levels.
I'll concede that the comparison samples I used were all studio shot under good lighting conditions so this will obviously have an effect on the ISO performance.
Under these conditions though I found:
At 6400 ISO the D810 was significantly cleaner than the D700.
At 6400 ISO the D810 showed similar levels of noise to the Df. Downsized the D810 looks cleaner.
At 6400 ISO the D810 showed similar levels of noise to the D4. Downsized the D810 looks cleaner.
At 12,600 ISO the D810 was significantly cleaner than the D700, particularly in the shadows.
At 12,600 ISO the D810 showed Very similar levels of noise to the Df. Downsized the D810 looks cleaner.
At 12,690 ISO the D810 maintained a little more detail but noise in the block colour sections was similar. Downsized then I'd say the D810 pips it.
At 25,600 the D700 is left well behind. Significant amounts of colour noise and bright spots.
At 25,600 the Df is VERY similar at full resolution, downsizing makes a big difference.
At 25,600 it's a close call. Personally I feel the D4 looks at tiny bit cleaner at full resolution but once things are downsized the D810 takes it.
The amount of detail the D810 holds onto even when viewing at the full 36mp resolution is amazing. The fact that these high ISO files are in the same ballpark as the D4 sensor even before downsizing gives the D810 an advantage.
All just my opinion based on comparisons from:
http://www.imaging-resource.com/