Bringing up teleconverters raises the question if any of these lenses in the patent can be used with a converter. Note how close the back focus at the short end is. The 100-400 is listed at just over 4 mm. A mistake? The other two lenses are close too, well inside the camera lens mount. It will be hard to make a converter for that.
My EF 100-400 ii pairs very well with the 1.4 iii even when it is used with the 5DS. There is a wee bit of edge softening which largely clears up at F8 and some CA. And the lens works well enough with the 2x iii for emergencies. It is best to stop down once for decent performance. An adapted R can focus the lens with the 2x, a bit slowly but accurately.
Given how well the EF lens can work on the R, I might prefer to keep using the EF 100-400 on the R because of the converters. It is a pain to use the converters with the adapter. There is always the chance of a fumble, especially while in a hurry with frozen hands. It will be too bad if converters cannot be used with the RF 100-400 from my standpoint.