Mmm, I think I would wait for a few unbiased reports on the AF than from a
Technical Advisor at Canon USA.
Do company men (not just Canon) ever say their latest product is not the best!
Odds are, because they have had an all electronic mount that things should work as good via the adapter, and hopefully better, but I'd wait for a few real world reports. Long lenses and the subjects they normally shoot, push the AF, face detection is one thing, but tracking a fast moving object is something else.
Long lenses also need to be powered, (I know all AF lenses do but big lenses are moving big pieces of glass) which mirrorless cameras are already having to work without having a lot of. Will that affect the AF, and will the big lens adversely affect the battery life! Again, so many combinations and variables that I doubt they have all been tested by Canon.
Of course to some Canon are infallible, I mean they are market leaders.