>I'm really just interested in how the D800 is likely to handle >DX lenses.
It is likely to handle DX lenses perfectly well. In DX mode, the D800 will generate a 15.3MP DX-format capture mode compared to 5MP DX file size generated by the D700. The overview at DPReview has a lot of accurate, hands-on information.
Not sure what you`re asking though if the foregoing is not the information you`re looking for.