I think we are probably talking about the same here but expressed differently. What I was trying to point out, in order to get the Depth of Field when in FX mode, you would stop down more on your aperture just to get a little more in focus which also affects your ISO unless long exposure is not an issue with static subjects.
When you use your D800 in DX mode, again as mentioned, the resolution is reduced but the advantage is you get the extra DOF (in focus) compared when shooting in FX mode. Basically with similar framing and aperture, DX will have more of your subject in focus compared to FX which is important when shooting macro.