Sonyalpha.blog tested the 300mm A-mount lens on the Sony A1 and writes:

The Sony 300mm F2.8 G II SSM A-mount II (7500$) is a lens with an optical design of 2003 (Minolta) and it shows its age. Sharpness is just “good” at F2.8 on 42-50/61 Mpix cameras. On 24Mpix it still delivers very good results at F2.8 . Things get much better at F4 / F5.6 but you don’t buy a 7500$ lens to use only at F4/F5.6

On A1 /A7RIV is works perfectly with the LA-EA5 adaptor although limited to 8fps : the Af is very quick and efficient. However the adaptor does not allow AF in video but all other mode works in photography : Tracking, eye Af, animal eye AF…

What is striking with this lens is the butter smooth background blur , that I only find an equivalent in the Sony 400mm F2.8 GM OSS or the Sony 600mm F4 GM OSS

You get some excellent shots with a superb rendering with this kind of 300mm F2.8 lens. If Sony could release an E mount newly designed 300mm F2.8 GM between 6000-8000 euros it will be an ideal lens for sports/action photography but also for wildlife in low light , medium distance , for concerts, Portraits from a distance ….

The LA-EA5 by combining the motor you could find on the LA-EA4 and SSM compatibility of the LA-EA4 offers a great and efficient way to still use with very decent performances your A mount lenses (preferably on 24mpix)

If you don’t own the Sony 300mm F2.8 , I would not recommend to buy it new , but if you find it used for 2000 euros max and are ready to use it at 24Mpix max go for it

Waiting for a E mount GM version of this lens , lighter and capable to support 61Mpix , and 30fps, you had a preview of what could be the rendering of such a lens