Some tests of 300mm GM with TCs too
The review of the Sony FE 300mm F2.8 GM lens highlights its exceptional sharpness, autofocus, and build quality, with notable performance for sports, wildlife, and fashion photography. It emphasize…
sonyalpha.blog
Looks like by his standards, 300mmGM + 2X performance is around the same level as 200-600mm at 600mm
at 300mm and 420mm (with 1.4x) its better than 100-400GM and 200-600mm which is not a massive surprise tbh.
This lens might be "the one" for me to keep me on Sony. I am pretty sure there will be an A1II which will bring all the improvements of A7RV to A1 (of course affording it will a be a 10 year plan

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I think I will wait see what this sigma 500m f5.6 has to offer first but being able to shoot at 300mm f2.8 and 420mm f4 is really attractive.