I'm hoping Sony does update the 400GM to a built-in TC version. When I bought my first Z9 at release I actually got a call from the head Sony rep in Canada who wanted to know what I was liking in the Z9. We discussed a bunch of things but one of the things we discussed was the built-in TC lenses. He gave me a shred of hope that Sony was listening and was looking into doing a lens with built-in TC. Years have passed but supertelephoto lenses don't get refreshed very often. So hopefully since the 400GM is the oldest lens it will be the one to get the replacement soonest and will have the TC.
For me it will probably a race between a much improved Z9II and a Sony 400TC.
I really hope there is substance to any intention on Sony's part to refresh the 400GM with a 400GM mkII TC, using the new lens technologies that helped getting the 300GM's size and weight down.
I personally have no desire to replace the A1 with the A1II, and my main birding lens is still the 600GM for every scenario except for dedicated long distance trips.
I recently went to Costa Rica on a dedicated birding trip, and came back with the intention to revisit the country and also make a trip to Ecuador. It was a good way to actually experience what kind of lens would work and what the limitations where of the lens I brought. I re-purchased the 200-600G and left the 600GM at home.
I came back knowing I would not want to bring the 600GM because of the need for access to a shorter focal length, for the lack of a way of fast packing and unpacking the lens with camera attached in situations where there's lots of (sudden) rain, and in general I feel I was right that the size is impractical in the surroundings I met.
The 200-600G did allright, thanks to the A1, but I will be selling it again and would not bring it again. The iso was climbing so fast and I was not accumstomed to that after 2 yrs of 600GM use. Also, the lack of GM signature color and detail was something I will always miss in a lens like the 200-600G.
It is a very good lens, especially for the money, but there was also the auto focus that felt shockingly impaired coming from the 600GM, being slow to re-focus, and often refusing to re-focus when going from far to near, having to focus somewhere close where it would re-focus, before actually being able to focus on a bird. Lacking DMF manual override made it all a bit awkward.
So having this direct experience, would I want a 300GM with TC's? Not really, but what I would buy on the spot for future similar trips, is a 400GM mkII with built in 1.4TC. Somehow, such a lens is now much more desirable than an update to the still fantastic A1, so I will be saving up for the event that Sony brings such a lens, hopefully using 300GM design signature.