People also need to remember that Nikon is achieving better noise performance (though perhaps not dramatically better) while doubling, and in the case of the D800, tripling the megapixels from the previous generation of cameras.
In addition, the ability to downsize a D800/D7100 image to a 12-16 MP one greatly impacts the appearance of noise, and how noise appears in photos can be just as important as at what ISO is starts to show. I am not bothered at all by ISO 3200 on the D800 downsized to 12-16 MP size with no noise reduction at all. I think when down sampled to the same size the D800 image with practically no noise reduction is still better than a 5D MKIII image, which applies increasingly aggressive noise reduction to the detriment of details.
As for the theoretical noise performance on the D400, I think recent DX releases in the last 3-4 years have shown just modest gains in ISO performance and I would expect it to be very close to the D7000/7100 in that regard.
However, as observed increases in dynamic range may in fact impact most peoples' photography in a much more meaningful way and I see the trend of better and better DR continuing.