I've decided to approach "the long question" another way.
After considerable research I think I've determined the lightest, fastest, sharpest, and cheapest way to produce 300 and 500mm images with an A7.
Reading many discussions by birders and other long lens users, the limits of the 70400 options come up fairly often. The next steps are the great primes. The latest AF versions of these are extremely expensive and heavy, though great optically.
I enjoy MF and so autofocus is not mandatory for me. Enter the last great MF Nikon telephotos, prized by both press and wildlife shooters in the almost recent past.
It so happens the lightest 300/2.8 and the lightest 500/4, are exactly these, and in the case of the AIS ED 500/4 P, the lens is only a hair behind the best in world today, and that in the corners wide open. It supposedly will take a 1.4x TC with no visible loss in quality, which adds a great 700 to your choices with a 80$ Kenko 1.4x.
I already have the 300/2.8 and the 500/4 is in the post.
I've really taken to the 300, which can be shot handheld with a fast enough shutter:
Mountain Beach by
unoh7, on Flickr
It's 2500 grams and the 500 is 3000 grams, mere feathers compared to other lenses that fast at those sizes.
A 300/2.8 will run $800 or so if you look around, and the 500/4 brings $1600 or so.
By comparison the latest Nikon 500/4 E, which is actually almost as light, is a mere 10K USD.
and the new Sony 500/4 G is only 12k