After I finished testing my lens, it occurred to me the method I used would probably give me a comparison from cropping to using an extender.
I had set the test up at the same distance (real world you would be using the extender or cropping from the same distance). Shutter speed remained constant at 1/500, all the shots are wide open and the ISO is set to Auto. This means that each shot has a higher ISO, with the exception of the 2x which I supplemented the light because it had become unusable. The 2x has a lower ISO than the native lens. The samples are cuts of screen shots hand scaled and zoomed in to be the exact same size (the exception is the Native shots which wouldn't scale precisely but were close), where I am now I didn't have editing software. All shots are with the R5. YMMV if you are using a lower MP body.

In the center what I am seeing is a benefit to use the 1.4x in the center. The Native crop is slightly better than the 2x, compound that with the loss of light due to the higher aperture I give the Native the edge. The 1.4x Crop is straight up better than the 2x. My opinion is that for any long work you would use the 1.4x and crop rather than use the 2x.

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For the Corners the 2x is weak. The Native might have a little edge over the 1.4x cropped, and it might not. It is debatable.

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At this point I see no reason to own the 2x. I can get the same results by cropping and not lose the light. For distance the 1.4x is the way to go.

As for the new lenses released, I think you would get fair results out of the 800mm but you loose light which is a big negative. The 1200mm MTF's show the lens as sharp as the 600mm with the 2x in the center. If that is the case the 1200mm is going to be a semi soft lens and for my use very limited applications.

Neuro did a comparison of the 1.4x and 2x EF versions vs the RF versions. It is a good read. Hope he doesn't mind, but here is his link: