Nate,

I have been shooting the R5 with my EF 600mm f/4 II and, as Brant noted, with a full battery I have been able to access the H+ drive mode with the mechanical shutter. I have not tested the frame rate but it doesn't seem too slow and the 20 fps with electronic shutter is available with the EF to RF adapted lenses.

This combination works very well for birds that are perched or moving slowly and the eye tracking AF performs great.

I have not used the 600mm for flight (due to size and weight)

I have used the R5 + EF 600mm with both the 1.4 and 2.0 TC .... both perform great with all AF points available.

Also, to use H+ drive mode with the mechanical shutter you must use the new battery version that came with the camera.

I am using a battery grip with 2 batteries and it drives the big lens just fine.

The older 5D batteries will work for electronic shutter (20 fps) and the slower drive mode with mechanical shutter but do not provide enough "juice" for H+ drive mode with the mechanical shutter.

As Brant said my son has a Version I 500mm f/4 but I have not tried it on the R5.

I suspect you would be happy with the 500mm f/4L I performance on the R5 ..... perhaps a rental before you buy would be helpful.

For birds in flight I am using the RF 100-500mm .... it is amazingly sharp for a zoom lens, on par with the EF 100-400 II. Not quite as good as a big prime f/4 but very capable of producing stellar images. It is light, well balanced and perfect for hand-holding with the R5. I let Brant shoot with it for a while and I think he was equally impressed.

Joel