I own a A6600 and mount my Canon EF tele zooms on to via sigma MC-11 adapter.
TheA6600 sensor is slower than A6700, and the AF is better in the A6700 to my A6600. I also own a canon R7 and have used both to take very similar images, ie sports, portrait, landscape and wildlife.
What really makes a difference are the lenses, and good light which everyone would like on most of there images,..lol
If you can fill the frame more with your subject and have bright light the results on both systems are very, very good. Both A6600 and R7 ( though R7 Eye AF is much better than A6600) struggle locking on with eye detect or subject tracking if the subject is small in the frame and low light. This is really a factor of having ASPC sensor mainly IMO