Dropping to 50 ISO means that very unusual conditions are present if 1/4000 is not fast enough. About the only time I see a need for 1.4 apertures outdoors in for portraiture and if the sun is that direct that requires 1/8000 at 50 ISO, I can sure bet the the shadows are going to be so harsh as to make the portrait unflattering. I shoot in those conditions a lot and seldom need even 1/4000 because some sort of diffusion is needed to prevent harsh shadows. The base ISO of 100 on the D600 is same using 1/4000 as with prior cameras that had base of 200 ISO at 1/8000. So, a D600 will handle very bright conditions with more headroom than a D700. Never heard anyone complain about the strong light shooting with a D700. Consider the D600 has 2.3 stops more DR at low ISO than the D700, I would be looking at technique before limitations of the camera if someone can't get their shot in strong light with wide apertures.