<I would like to see the same comparison with both camera images taken in DX mode>
The center of the frame will show the same relative result. You are just taking a crop from both cameras.
Both cameras in DX mode show more noise if you are comparing to FX images. The impact of going to a smaller size image with the DX crop means you then have to upsize the D700 DX crop which magnifies the noise. On the D800 DX crop, you lose some of the beenfit of downsizing since you are starting with a 15+ mp image. The best indicator of the D800 with a DX crop is a D7000 - same pixel pitch, DX , and same processing.