The gear arrived a day early (thank you lens rentals!) so I got a little time with it in the back yard before it got too dark.
The first thing is, of course, weight. The a6400 (sd card) + 1.4x + 100-400 is only 2219g, compared to a6400 + mc11 + siggy 150-600 C at 2649g vs d850 (xqd card) + grip + EN-EL18c + Tammy 150-600 g2 at 3594g. That 1.0 - 1.4kg is very noticable.
The a6400 body is, for me, almost too small for my hand. I might add the vello grip just for more to hold on to, though with a big lens with tripod foot most of the holding is in the left hand anyway.
I shot mostly with the Sig 150-600C. it did not feel too off balance and handled pretty well. Focus seemed a little slow, but that might be pilot error until I get all the a6400 settings figured out. It was also just before sunset so the lighting was not too bright. I also need to sort out AF-C vs DMF vs various zones vs tracking. I didn't see a built-in help button to describe a setting like on the Nikon.
One of the biggest problems with the a6400 was the viewfinder / monitor switching. I would see a BIF (crow) and bring the camera up to my face and would miss the bird while the VF/monitor switch. I assume there's a setting to force it to VF only, but I could not find it.
Another issue with the EVF is that I have a hard time finding out-of-focus subjects, like going from near focus (say 10m) to BIF at a distance. It all gets blurred out. I also have this problem with other EVFs, not just the a6400. I find the optical VF easier in that regard.
The subject tracking seemed to work pretty well, but I didn't have much to test it with. I look forward to trying it on some sea birds tomorrow.
I mostly tried the 100-400 when it was getting pretty dark. With the TC on the focus was slow and hunted. Without the TC, it was pretty fast and didn't hunt. I'll need to try it with better lighting with actual subjects not just kitchen boxes.
I'll keep posting more updates as I get more experience with the gear. I've not even looked at the photos in lightroom yet.