No matter how good a telephoto is with structures and static subject matter, my main criterion is how well it can capture bird feather details. Reading half a zillion reviews over the last decade plus, the feeling I get is that primes are generally first choice, followed by a prime with a teleconverter, zoom, with zoom plus teleconverter taking up last place.
New releases over the past few years, though, have upended that pecking order; as newer designs, often synced with software corrections, have produced some eminently capable configurations of all types.
Today I managed to find a couple of feathered friends to sit for shots, and you can be the judge of where the RF 100-400mm paired with the RF Extender 1.4X fits on the totem pole.
My opinion? Further research is needed, and I expect to enjoy researching!