D600 High ISO in DX

The D5200 isn't out yet AFAIK but you can get an approximate comparison by using DPR's samples. Go to the studio comparison tool and choose the D600 and also the camera you want to compare it with. Download the files (I would use the RAW versions). Reduce the other camera to approx 4000 x 2667 size. The frame of the two photos isn't the same but the noise per area is what you would get. I assume you are interested in noise for the same frame/size.