I do not plan to use the D800E primarily as a DX camera. I plan to buy the D800E primarily because I want the 36MP performance in FX mode.
I realize that the performance of the D800/E in DX mode is not equal to the rumored specs of the D400. I also realize that Nikon may or may not produce the D400. At this time I am considering whether the D800/E might be a "reasonable substitute" for the D400 since the D400 is not available. If Nikon does produce the D400 (or whatever else Nikon may choose to call it) with specs that are similar to the rumored specs I would then also buy the D400.