I've owned a 6D and have seen other 6D's been used in motorsport. The issue I got and also the other guy is not about IQ, it is about keeper rate. I may get 2 out of 10 that are tack sharp while the other guy did a little better around 3-5. Keeper rates are what keeps us in business unlike portrait shooters.
Well speaking as a user of both cameras... The 6D's AF really isn't bad at all. In one shot I find the 6D to have more accurate AF than the 7D, however the 7D is more accurate for AI Servo/sports.
The biggest problem though is not the AF keeper rate, it's the pathetically slow 4.5 FPS rate of the 6D. If you're shooting a fast moving game like rugby, the perfect shot is just a split second. With 8 FPS the 7D gives you much higher odds of capturing the perfect split second.
Another thing is that the APS-C crop is incredibly useful for sports. To get similar reach as I do with my compact and fast 200mm 2.8, on the 6D I'd need a hugely expensive 300mm 2.8, a big heavy lens that needs a monopod.
I'm just waiting for a 7D Mark II that can produce usable results at ISO 6400 for night sports... but with a 2.8 lens ISO 1600 to 3200 is usually enough to maintain a shutter speed over 1/1000 in a bright stadium.