>I have taken some DX mode shots of surfers with the 70-300mm >VR trying to "fill the frame", whereas in FX mode I >could not do so, obviously the shots were from the same >distance. It SEEMS that the DX shots were more clear, and at >any rate, did not suffer. Same apparent result with some >racing sailboats. These shots were at distances I could not >"reach" well with the FX mode and 300mm lens.
This suggests that you were racked out to 300mm in FX and may have been able to use a slightly shorter focal length in DX. On the D600, my 70-300 is significantly sharper at 250mm than 300.