The increased efficiency of the IS in the RF100-500 compared to the EF100-400II is very noticeable, especially when coupled with IBIS. But as you say, if you have the EF100-400II already, the RF100-500 is not as tempting as when you lack coverage in that range.
I don't doubt the superiority of the RF 100-500mm. The stabilisation efficiency of RF lenses in combination with the R5 is at least 1-2 stops better than EF glass on the same body. However the stabilisation on the EF 100-400mm is more than adequate for my usage, as I'm primarily using it for active wildlife that would require 1/500 or faster to freeze subject movement.
So for me, considering that I also use a 5DMkiv alongside my R5, it makes more sense at the moment to keep the EF model. It's all a question of priorities - I can get stunning images with the EF 100-400mm, and while the RF 100-500 may be better, I'd rather spend my money on travelling. For me, ownership of the latest and greatest gear is very much secondary to being able to travel extensively around the world and enjoy photographing wildlife, so that's where my money goes.
The cost of switching to RF 100-500mm, plus extender, plus the necessary purchase of an additional RF body, is roughly equivalent to the cost of 2 African safaris, and I'll take the latter every time, in preference to buying a modest improvement in lens performance.
Of course, if I was lucky enough to be able to keep on travelling to the same extent, *and* still have thousands of dollars in my back pocket, I'd sell the 5DMkiv, get an additional R5 body, and buy a multitude of RF glass including the 100-500mm, the 14-35mm, the 400mm F2.8 and both the 1.4x and 2x extenders. Or I might change systems entirely.
I use an EF 100-400 II with 2x tc and R6. I like having the full range of 200-800 for taking pictures of my kids and birds without having to change lenses/tc. Does anyone with the RF 100-500 miss having the full range with TC attached? Besides the $$ cost of switching, this part keeps me from considering the upgrade.