Is the APS-C DOF penalty all that important?

...APS-C has no "extra DOF" over FF, as the FF photographer can simply stop down.  In an equivalent situation, where the photographer needs the same DOF and shutter speed, the FF photographer simply gives up the noise advantages that FF has over APS-C (barring, of course, the situation when an APS-C camera has a more efficient sensor).

Again, I would disagree.  The 7D, for example, has nothing on the 5D3 in terms of DOF or AF, although the higher PD (pixel density) of the 7D sensor does give it a reach advantage for a given focal length.