My 5D Mark III is here and I've had a very brief chance to compare the autofocus performance with the 5D Mark II. I'll be doing some flight photography this weekend, but in the mean time I tested out my other use case for pro AF, photographing with large aperture primes in low light. I put a 85L on both bodies and went to photograph my 3 1/2 year old daughter in the living room. She was sitting about as still as a 3 1/2 year old tends to sit and I tried both the center and outer AF points (using AI Servo). I took a series of images at full speed with both cameras. I then processed the images, cropped them down to the point of focus and made a movie out of the frames in Photoshop CS6. Here are the results.
That is the 5D Mark II using the center AF point which is much more accurate than its outer AF points.
This is the 5D Mark III using the center AF point.
Now this is the 5D Mark II using the left-most AF point.
Finally this is the 5D Mark III using the left-most AF point.
This isn't the most scientific of tests so you are free to draw your own conclusions. For me, one of the things that drove me bonkers with the 5D Mark II was flaky AF with the outer points when working with fast primes. I am thrilled with the performance I'm seeing so far with the 5D Mark III in this regard. Whether this will translate to better tracking performance for avians and wildlife remains to be seen.
The exposure I believe was around f/1.2, ~1/80s, and ISO 3200. Thus the light level was around 2 EV give or take. Its not exactly candlelight, but not bright either.