One thing to keep in mind about Nikon and purple fringing is that Nikon uses several chip makers in their cameras. Some models sport a chip from Aptiva. Some from Sony. I believe there is a 3rd company as well. The D800 has a Sony Exmor sensor.
Nikon has input, how much is unknown although Nikon also make lithography gear for silicone chips and sensors so they must have a deep knowledge base.
I've also recently started experimenting taking long exposures with my D800E. In the darker areas I've noticed lots of hot pixels. Is this a common problem with the D800E?
Noise in dark areas in long exposures at high ISO is a problem with almost any digital camera. Some are better than others but most these days are pretty good. The kings of long exposure are the Canon 6D, the Sony A7s (probably the best), the Nikon D4s, the Nikon DF. The Canon 5D3, IDX, Nikon D610, D800, D800e, Sony A7, Sony A7r, Fuji XT1 are all low noise high ISO machines. But they will all have some hot pixels in dark areas to a greater or lesser degree.
These days the manufacturers do a lot of in camera noise reduction and there are several levels of noise reduction cooked into the firmware that may not be entirely controllable by the user but generally noise reduction is best done in post processing not in camera where it can damage stars, give a plastic look etc.
I had a D800e for a few years and now have a Sony A7r which is basically the same sensor (Sony Exmor 36.4mp full frame).
Basically noise performance is about as good as it gets from any sensor or close to it. All these full frame sensors tend to need noise reduction in the processing. If you image on a warm night it will be noisier.
I would use long exposure noise reduction set to on as the D800e has a small amount of amp glow (purple tinge in the lower parts of the image), the Sony does not so its something to do with the Nikon circuitry.
30 seconds ISO6400 long exposure noise reduction on, F2.8, 14mm or so, RAW, white balance to about 4200K, vivid is a good formula for this camera.
For the Sony or Fuji cameras auto white balance works better (they seem to have a superior white balance engine to Nikon). The noise pattern in the D800e is a bit nicer than the Sony (a bit wormy).
I have taken 1 hour exposures with a D800e and they were stunning.
