I was just testing my D800e today with tripod and 500mm with test target at 75 ft with your question in mind. I took jpegs and viewed side by side images at actual pixels. Of course you could improve the acquisition technique if you took raw files without sharpening. (In camera, sharpening set to none).
I reviewed unsharpened jpegs and also did noise correction and mild sharpening in LR 4.3, as processing in LR is my workflow. (I typically have a RAW not jpeg workflow but I didn't want to make this too complicated).
In any case, I thought noise became personally bothersome(acceptable is subjective) and not easily corrected at ISO 2000, without beginning to get a plastic appearing correction. That is to say, ISO 1600 had essentially no noise without any loss of sharpness.
Not sure if 800 is same as 800e but it was very helpful to correct the color noise as the ISO went up. In my tests there was some magenta showing up in areas of gray, easily corrected.
What you can get away with is different than what is ideal. I found ISO 3200 no problem at all for reproducing posters.
My ideal for the 800e is ISO 200 for best low light contrast. I found details best at ISO 400. I agree with what is written above. David