>ONE QUESTION LEFT: could it be that in DX mode the autofocus >points, which are then "larger" have an easier time >focusing than in FX when the FX shot is at such long range >that serious cropping is necessary? I ask because the surfers >seem more clearly in focus in the DX mode shots. ISO, aperture >and shutter speed are the same.
In DX, it is true, the focus points are 'larger' in relation to the capture area. But, big but, nothing else has changed IF you are using the same focal length whether in FX or DX crop mode. The size of the focus points relative to the pixels underneath them or the scene illuminating them has not changed.
Any difference in sharpness (that is to say, focus or, equally likely, enhanced motion blur) is again going to be more dependent on focal length and its attendant issues (motion blur, depth of field, lens quality to name a few) than anything else. VR is wonderful but it is not anywhere near perfect and of course it just accommodates camera motion, not subject motion. Vibration reduction, not elimination.
I have been using the 70-300 on my D600 as my go-to lens and it has been at pains to teach me a lot about the need for a very steady platform when set out to its limits. With a tripod or monopod, I can enhance image sharpness at long focal lengths substantially over handheld, especially in failing light. Even in bright sunlight, shooting at 1/200, f9, iso 100, VR engaged, there is a substantial boost in reliability of sharpness to be derived from steady support.
Another capability that the D600 opens up to all long focal length shooters is to raise iso without significant IQ penalty in order to get another few stops of shutter speed and aperture.
While all of this is true, I certainly do not shy away from 300mm on this lens. I use it a lot and, if I am forced to shoot handheld, depend on its very good VR to really save my bacon.