All my lenses are FX. Your point about the viewfinder is a good one. I do not have a digital FX camera so I have no experience with using one in DX mode.
As I recall, the D600 is capable of about 5 fps vs. 4 fps for the D800. I doubt that in most applications that small difference would really matter.
Of course, what I really want is both the D800E and the D400. I don't want to buy a D300 due to the age of the technology and the probability that it will be replaced by....well, something. I don't want to buy a D600 since I want the D800E. At this time Nikon is not making the camera that a lot of us want, which is a camera that has approximately the specs that are speculated about the D400. I would be happy if that camera had less than 24MP. At this time I am just trying to figure out how to bridge the gap until (perhaps I should say if) Nikon makes the D???.