It looks like I will be getting next week an R5. I do have two very good copies of the 100-400mm II + 1.4 and 2xTCIIIs and was intending to use one of those. I do have the opportunity of buying an RF 100-500mm, which is a serious outlay, especially with the high cost of RF extenders. I'd love to hear reports from those who have compared those lenses on the R5.