This is what I discovered.
I was very frustrated with the focus, especially in AI Servo, when shooting birds against busy, branch-heavy backgrounds. Focus hunting was awful on my 60D. After switching -- as recently as a few months ago, I woke up -- I'm back to considering a 6D as a viable FF option, though the center point is inferior to my 60D on slower than 2.8 lenses. Depends on how much Santa and my employer's Bonus Fairy brings.