I did a lot of tests recently, comparing 5D2 with 5D3, for Advanced Photographer magazine.
On AF performance, shooting multiple runs of a motorbike heading straight towards the camera at 50mph, the 5D3 was 3x more likely to get a sharp shot. Typically a 90% spot-on hit rate was double the 5D2, especially when the bike was near and closing very quickly, plus it shoots 50% faster. Servo AF with centre point on release-priority, same 70-200L Mk2 on both.
Off-centre AF performance, 5D3 wins hands down. Canon has done a fantastic job with the 5D3, basically if you miss focus with an L-grade lens, it's either technique or wrong AF settings selected.