The crop is pretty straight forward, but nobody has mentioned the impact of the number of focus sensors available or the relative size of the AF sensor compared to a DX camera.
The D800 has 51 AF sensors across the full frame - they cover about 70% of the image horizontally. But when you are in DX crop mode, there are 11 AF sensors covering 90% of the frame - placing some sensors outside of the normal composition. The D300s puts 51 focus sensors on a crop frame - 11 sensors horizontally covering the central 75% of the frame.
In relative terms, if you are cropping to DX mode the FX AF sensor is going to be slightly larger covering a less specific area of the image. In other words, the sensor may cover the head of an animal rather than just the eye.
Now I'm not sure whether this is actually the way it works in the camera. My comments are based on illustrations from the respective manuals.