Supernice lens, works well with extender EF 1.4x II giving you total 560mm.
Even with the extender autofocus works on EOS-R (it shouldn't according to the manual). Of course EF-RF adaptor was used.
Personally I find the results are a bit sharper. Also just the lens I find better balanced. I also have the much lighter RF 100-400 and use the 100-500 when I don't have to carry it too far. It is also my wife's lens so another consideration!