I didn't have to make any focus adjustments to mine.
I am not and would not tell you to not buy the dock but you may never need it. If the lens is doing what you want and need it to the dock won't make any difference.
One big misconception about super telephotos is your ability to shoot small subject, like birds, form a great distance. This is totally false. A small bird shot at distance even with a super tele will still be a small bird in your photo. The best use of a super tele is to increase the subject, a small bird, size to more fill the frame. However, this requires you to be at or very nearly at the same distance as you were with your 300mm lens.
There usually is no magic bullet in photography. And, used incorrectly, you may not, will not, see much improvement with the new super zoom. The on point advice of, to get closer to your subject is still the main most thing. The reason I mention this is, I get the feeling you thought the big Siggy will make shooting birds from a greater distance possible. It won't!
It will have a learning curve so take your time and don't get discouraged if the first shots are less than perfect. Keep in mind you are dealing with a lot of focal length ay 960mm. That is huge FL in anybodies book. As magnification increase so does all the other photo destroyers by the same amount.
EB
EOS 1DX and 1D Mk IV and less lenses then before!
