To look at your question specifically, noting that at this stage no-one that I know has a full production model yet.
The R5II two has features that may benefit a wildlife photographer through its bus: It has a stacked/BSI sensor that is going to grab focus extremely quickly, provide enhanced tracking and reduce rolling shutter effect to support faster shutter speeds and frame rates under more challenging light conditions with better dynamic range.
The processor is faster and has a significant boost from the extra attached processor, the buffer is likely larger and there is the capacity to send RAW image to the CF Express card. Assuming this is all in balance (i.e. no bottlenecks in data flow) it means that one can gain focus, shoot with more accuracy and rip off a lot of high-resolution images with a much higher success rate than before. Wonderful, as long as you are prepared to spend hours sifting through a huge number of almost identical images in PP...
I would be curious to see if the ability to prioritize a series of individuals, which is touted for human social events, such as weddings, would be made to work to isolate, say, a predatory animal group or individual hunting in a herd situation such as a pride of lions or even an orca pod. That would be interesting...
The question is whether the demands of the specific types of photography one is engaged upon are such that the R5II presents enough real and specific benefits to make it a valid value proposition, or could one get more value by getting the R5 and spending the money on an RF optic, such as the RF 200-800?
Finally, considering some discussion has been made of the relative merits of the R1 in this context, I submit my updated article The Argument for Two Flagships.docx for consideration.
As to lenses: I would expect that, as long as you use the Canon EF-RF adapters , you should have no issue with using your excellent EF 100-400LII with any of the R-series of cameras. I have already posted sample images of that lens with the EOS R6, which has the same tracking system as the R5:
Legacy Lenses with the EOS R6 MkI: EF 100-400L MkI... - Canon Community
cheers, TREVOR
The mark of good photographer is less what they hold in their hand, it's more what they hold in their head;
"All the variety, all the charm, all the beauty of life is made up of light and shadow", Leo Tolstoy;
"Skill in photography is acquired by practice and not by purchase" Percy W. Harris
