As I stated above I’ve got the 400 DO IS ii, I’ve also got the RF 100-500, both I’ve had hooked up to the R and R5. Both have superb sharpness, RF auto focus has been stellar, 400 is stellar when it doesn’t jump (hopefully much better with the new firmware I just plugged in). The obvious measure for me is the 7.1 max Ap on the RF. If you shoot wildlife in any setting other than well lit prairies/fields/savanna’s/marshes, it’s unacceptable, too slow. Any area with foliage or shading, and your asking for your subject to pose and not breathe. The 400 DO ii still is the workhorse, with the 1.4tc living on it, best BIF or quick shooting, close “combat” combo for mammals you can get, if the price tag is manageable, IMO. 560mm at 5.6 thats super light weight and easy hikeable on a Cotton Carrier. Yeah, the RF zooms, but those of us looking for that reach in a quick maneuvering lens, those are the options from the Canon L’s. We’ll see what Canon can muster up in the RF line to rival the 400 DO ii, till then, nothing can touch it for those purposes.