The D600 uses new Sony sensor, which Sony representative insisted to be absolutely the best now. I did a little, quick and dirty comparison with D4 and D600 seems to beat this thing. It's dirty, because raw editor I have used doesn't have full D600 support, in particular it doesn't have proper linear rgb color space transformation matrix, so colors are a bit different. Proper color matrix will equalize colors but may introduce some more noise (it also may decrease noise level instead, though). First, 24Mp > 16Mp, so we have some room for improvements before downscaling an image to compare. I used this possibility in ISO 25600 by applying light NR.
The ISO 6400 sample was downsampled as is, without NR applied.
First, ISO 25600:
Full samples are here. The D600 isn't fully supported yet, so you'll see artifacts at the very right. Not a big deal though.
IMO Nikon is trying to defend D4 positions, so we have such a crappy JPG engine for D600.