@ntz I really don't care what you think about it, but my reality
IS that I can't shoot at ISO 800 on my D7200 without noise filtering to the point of losing image detail, and I can produce very good images from the D800 at ISO 3200. I know what I see from my own cameras, on my own computer, so don't tell me that it's nonsense. As for the D7200's performance, I was amazed by it after upgrading to it from a D7000. Nevertheless, I find it unacceptable at ISO 800.
Also, your comparison of ISO to crop factor makes no sense. That's no different than saying shutter speed is different on a crop factor camera, or aperture is different on a crop factor camera. Exposure is exposure, nothing changes from sensor size. Depth of field at a given aperture is different between DX and FX, but exposure is not. Some have said here that less light gets through a DX lens, which may be true, but it's not less light per area of the sensor, and unless your metering emphasizes areas in the vignetted area, there's no difference. Also, many DX zooms are vignetted only at their widest range, and
will cover an FX sensor at the long end. (My DX 10-24 covers the FX frame at 15mm and longer; vignetting only appears under 15mm.)
And I still don't know where you're coming from with your view that a camera is somehow crippled by mounting a DX lens. There is some
very good DX glass out there. That said, I would never consider mounting my kit 18-55 on the D800, under any circumstances, but my work with the 10-24 on the D800 shows very nice results.
When I got the D800, I made a couple of test shots to compare. These shots are 1-to-1 crops of a sections of the frame, shot at ISO 6400 on both the D7200 and then on the D800. No filtering has been applied, and the difference is quite obvious. I don't have a similar comparison handy at ISO 800, but my test that day was to see what the D800 would do compared to the D7200.
As for losing something by mounting a DX lens, yes, you lose resolution. The D800 is 36MP in FX, 16 MP in DX. So in DX mode it's not quite the 24MP I get on the D7200, but it matches the 16MP I got on my D7000, and has far superior low-light capability to either of those cameras, as expected for a top-line FX camera, even though it's older than the D7200. I'll take a 16MP usable image over a 24MP throwaway image every day of the week and twice on Sunday! So please, please, please, tell me what I lost. I'll tell you what I
didn't lose, and that was the images I shot on a Saturday evening a week ago with my DX lens on my D800, which I would not have gotten using the D7200.