Two of the major area of improvement with the 5D Mark III are the AF system and high ISO performance. I went to the House of Dancing Water Show in Macau few days ago and tested out the 5D Mark III under some rather adverse condition and I think it fared rather well. I used the Canon EF 70-300mm f/4-5.6L IS USM with the 5D Mark III. The photos were shot in RAW and converted with Capture One Pro with resizing and sharpening in Photoshop CS5. I will just post 2 shots.
The above photo was shot under extremely low lighting, you could hardly see him in real life. The performer was hanging in mid-air swinging and “flying” around. It was not really a good photo and I posted it to show that the 5D Mark III managed to track focus and took a shot at all was quite an amazing feat. It was shot at 300mm 1/200s f5.6 and ISO 25000.
The above shot showed a stunt motorcyclist “flying” in mid-air. Again, the 5D Mark III AF had no problem tracking focus. The shot was taken at 252mm 1/20s f5.6 and ISO 25600.