It is a great lens, I have made good use of it with and without the 1.4TC. I also had the 100-400GM lens. After getting the 200-600, the 100-400 served no purpose other than acting as a semi-macro type lens for flowers, butteries, dragonflies, frogs etc. I did controlled testing and found that at 400mm both lenses were essentially equal in IQ. Comparing at 600mm (or 560mm) on the 200-600 vs 560mm on the 100-400/1.4 TC there was a clear difference where the 200-600 was sharper. I found myself never picking up the 100-400 anymore (except for the occasional semi-macro stuff in my backyard when I was bored). I eventually sold my 100-400, but to be fair that was also influenced by buying the R5/100-500 which would do any semi-macro stuff I cared to do anyways. I may not have sold the 100-400 if not for picking up the Canon kit.
I still think that the best value in a MILC bird photography system (and I've owned just about everyone out there worth owning) is the A9 and 200-600 and 1.4TC. Like Steve mentions I had issues with the A7RIV and 200-600 so I found more success in putting the 1.4TC on the lens on the A9(A9II) and making up most of the reach instead of using the lens on the A7RIV. I did eventually find a set of settings that gave me decent results with the A7RIV/200-600 but I still wouldn't recommend it as a first choice. I see no reason why the A1 won't be every bit as good as the A9 with that lens and should be better if the specs correlate with the extra performance one would expect from reading them.
I mean we have 1.5x faster sensor read speed providing 2x AF calculations per second (120 vs 60 in the A9) and that is compared to the already excellent A9. Compared to the A7RIV that does only 20 AF calculations per second, the A1 is now doing 6x more calculations. I can't imagine a reason the A1 wouldn't be excellent with the 200-600...but never say never. We will have to see for ourselves or maybe get a better idea once the review embargo ends this Thursday.
Just a few examples of A9(II)/200-600/1.4TC shots:
March 29, 2020.jpg by
Bird/Wildlife Photos, on Flickr
February 14, 2020-2.jpg by
Bird/Wildlife Photos, on Flickr
February 02, 2020.jpg by
Bird/Wildlife Photos, on Flickr