Shooting the RF 2X on the 100-400mm most of the time today, it appeared to be the best doubler I’ve ever used—no veiling flare whatsoever wide open, with good details on most subject matter. That said, acuity on contrasting edges is just a smidgen less than what the RF 800mm can do, likely attributable to the extra stop that goes further past the R5’s diffraction-limited aperture of f/7.1.
Where this extender really excels, though, is near MFD. It helps that I can get critically sharp images down to 1/100 with electronic shutter, though 1/400 seems to be the sweet spot for consistently, perfectly stable images. While default deployment of the lens in good light will continue to be with the 1.4X, the 2X will surely get a lot of use in certain shooting scenarios.
AF in decent light is plenty fast, and critter eye AF works as well as with the lens alone.
Much to like!