Just an update on the tests. I've done tons more with different speed lites, targets, etc, most of this has been 6D vs D600 type stuff. Ive even recorded video of how I do the tests so you guys can see and I will post the video here when it is done.
I am not seeing any difference between the 600 or 580 Speedlites. I did look into the auto focus settings on the 5Diii, but as another forum reader pointed out, most of it has to do with AI Servo type shots (which doesnt require a focus lock) to take the shot.
I should probably add a note, as I did on the Canon Forum about why I am going back and forth between the 2 differnent targets at different distances; Target 1 serves mainly to defocus. If im just aiming at target 2 and focusing over and over, its not a good measurement of the cameras ability to get that initial focus.
Another point is that the average focus times for each shot work out to:
5Dii - 1.5s
6D - 1.7s
But this is not what is happening. The focus lock on Target 1 (in good light) is always very quick, almost instantaneous.
Focusing on Target 2 is taking longer than these average times. In fact, most of the time is spent on Target 2.
It is hard to be precise, but I would say the times are closer to:
5Dii - 2.2s
5Diii - 3 + s
I have a Sekonic 558 Light Meter that seems to be unable to measure the light Im shooting in which would be ideal to know, and am waiting on my new meter - that should help to know exactly at what point it starts to struggle.