Now that you bring up that point, and seeing how the 6D is $1500 on B&H, I'm very tempted. Even though I do Concerts, I feel as though there isn't AS MUCH movement as something like wildlife/sports which I will never be shooting. So as long as the 6D can be a competitor for Concerts, I think it might be the way to go.
It should be fine. Lateral movement at a far distance is not challenging AF-wise. What is hard is when the target is moving erratically and is closer. I used the 5D II for kids soccer and the AF worked OK. Is the 5D III better? Yes, but I got plenty of good images with the 5D II, and the 6D is more accurate than the 5D II. Using the 5D II with fast primes with the outer parts when the subject was close (thin DOF) was asking for trouble. The outer focus points on the 5D III are much better. The 6D's aren't as good, but they're better than the 5D II. Part of the photographer's task is knowing the limit of his equipment.