#2. "RE: D800E Center-Weighted Metering in DX Format" In response to In response to 1
Light falloff around the edge of the FX frame can be observed with some DX lenses. If brightness levels over the entire FX frame are used in the exposure calculation (in either matrix or center-weighted), then it seems that the light falloff would be a source of metering inaccuracy with DX lenses on an FX camera.
I have not thought about this question before and have not used a DX lens on my D800E. It may be that the effect on the overall exposure is not enough to matter.