Basically the greater pixel density will allow you to crop further, but it will also put greater requirements on your glass and your technique. Which is to say, 300 f/4 + 1.7TC might not allow you to take advantage of all those pixels.
D7000 has nearly the same pixel density as D800 and comes pre-cropped for a couple thousand dollars less and is available today. You could test your theory with a rental of that camera. If your only purpose to buying the D800 is to crop your crocs, then just buy the D7000 and save thousands twice over. DX shooters have been shooting these kind of pixel densities for a couple years. Nikon V1 allows even greater pixel density than either of these cameras.