I can speak from experience, I own the RF 600 F4 for almost 2 years now, use it for wildlife and I am over the moon happy with it. I can tell you when I shot with the R5 the focus was not as fast as it was on an R3. It did not utilize both focus motors in the lens. It was quite noticeable.
I have since upgraded to the R5m2 and the af moves as fast as the full bodies (R1, R3). So a great upgrade from the original R5, for wildlife the biggest improvement is tracking with the R5m2 over the original. BIF are a breeze now. The original was definitely not using the full potential of the RF600.
I have many perched birds in very low, late light, at 1/60 of a second with the IS + IBIS on a monopod and surprised at the hit rate I get for sharpness. This lens always amazes me.