no i haven't but i think a pitch black room with no lights on in the middle of the night is pretty much the same thing as far as lighting conditions go.
From what I read the distance of the object lit by the af assist seems to have a significant impact on the focusing ability, next to the lens used... and your "pitch black" setup might sound like a worst case scenario, but not that's necessarily true because in real life the camera has to deal with varying combinations of af and real light.
good point. I was focusing on something about 5 ft in front me. I was able to see the AF beam through the camera because it was so dark and the red really stood out.... and yes, when I say pitch black, I mean pitch black. no street lights shining in through a window... nothing. pitch BLACK.
so anyways, i tested again with as much distance as I could in the same room... the longest dimension of the room is about 30 feet so I just pointed at the wall 30 ft away, even though i couldn't see a damn thing, waited for focus, and shot.... AF worked fine, like I was in a well lit room. the wall was far enough away that I sure as hell couldn't see the AF beam on it. Focus was fine.
I have other thoughts on variables that might be tripping people up:
1) what lenses are we having issues with? I used both the 24-70 2.8 II and the 70-200 2.8 IS II... both at 2.8 when I did my "testing". Maybe f/4 lenses are having a harder time seeing the AF beam in low light.
2) Also, would the color/pattern of what you are focusing on in low light contribute to the camera having a hard time. For example, if the subject was RED in color, or even had something with red tones in it, maybe there wasn't enough contrast between the subject and the AF beam from the 600.
3) Maybe the problem is in the speed lights and not the 5D mk3. Low batteries maybe? maybe without fresh batteries, the AF assist beam isn't bright enough. or maybe some of the new 600 ex-rt units (remember those are new too) have issues
I'm just trying to find a way to re-create the issue that people are having. There's obviously enough of a problem (or two) with this combo that a lot of people are noticing a significantly slower focus speed. I for one cannot re-create the problem. I swear, it's just about as fast as if i was focusing in broad daylight.
Can any of the other people who have the issue reply with the lenses they've used, the subject/color they're shooting, and try it with different speed lights with fresh batteries?