I am looking out of the window on the 17th floor of a Shanghai hotel, with the cityscape shrouded in mist and evening coming in at 5-15 pm. There is virtually no contrast between the grey spikes on top of the buildings and the grey mist. So, nothing better to do I have tested A1 servo on the 5DIII and the Tamron.
1. Set to centre spot focus, A1 servo and 600mm, the lens locks on very quickly to a very thin pole on the top of a distant building and instantly refocusses as I pan to closer and closer objects and back again. So absolutely no problem.
2. Set to the 9 centre points focus, A1 servo and 600mm, the lens locks on much more slowly to that very thin pole.
3. Set to all points focus, A1 servo and 600mm, the lens locks on even more slowly to that very thin pole and struggles to refocus. So, there is a problem with this setting in these extreme conditions.
But, I recall that with the 300mm f/2.8 + 2xTC @600mm, I have been sometimes been disappointed with the 9 centre points focussing. So, there could be problems under certain conditions when tracking with all points, but this is probably not unique to the Tamron.