I have been using the Canon mk I for several years and am very happy with it.
When I originally went to buy the lens I looked up every reference and review I could find, including comparisons vs. third-party lenses such as the Tamron. In the cases where they mentioned autofocus at all, the description read something like "Nearly as fast as the Canon" or "focuses/tracks almost as well as the Canon". I never saw a single review which said "autofocuses faster/better than the Canon". That was enough for me, because given that I shoot sports all the time, I did not want a lens which was "almost fast enough". Indeed, I've never been in a situation where I thought "this lens focuses too quickly/accurately".
On the contrary, I'm always looking for a little bit of performance improvement and am about to borrow a mk II just to see if it's enough faster than my mk I to justify an upgrade, for myself. I've heard good things but haven't compared them.
That said, perhaps the Tamron would be entirely good enough for most people and I don't want to criticize it because I've never used it. But that's where I'm coming from.