I use the R5 with the RF 2X on an RF 100-500 quite frequently and that combination can provide some quite acceptable images if you are careful about it. Of course, less magnification with the RF 1.4X is a bit better if you aren't in need of the extra reach. It all comes down to balance, IMHO. If you need the shot and the subject is too far away, the RF 2X is a definite option. If you can shoot with a prime lens and no extenders, that's great. Different tools for different situations is the rule. These full-frame images were shot in our backyard with the R5, RF 2X, RF 100-500. For my purposes, they are very usable. If I want to get out the EF 500 f/4 II and mount it on a tripod, I might do better, especially with the EF 2X III extender which is quite sharp with the EF lens.