I have only used the RF 100-500 (borrowed Joel's for awhile at Laguna Seca). So most of this is based on a review of specs, and reviews. Just to get this out there, but a common reason that will apply to all of these the fact that I still have the 5DIV as my second body and my wife shoots an 80D, and while she has a EFs 15-85 and a EFs telephotos but still borrows my EF glass, especially the telephotos, when she shoots. So I am not ready to exchange EF for RF. At this point, it would be in addition too in many instances.

The RF 100-500 is slightly sharper, gives you that extra reach, but the big selling point is that it is lighter in relatively the same size. I'll almost certainly upgrade to the 100-500 at some point, unless something faster/more reach but still versatile gets released. Was it "substantially" better? It was substantially lighter, that was noticeable the second I swapped lenses. Bryan's "in-use" weight is only 90 g different, that doesn't include the converter, so ~220 g w/ converter. I do wonder if the weight of the 100-500 is better balanced as it felt like more than 1/2 lb. The bit more reach was noticeable. In terms of IQ, I've seen comparisons, I can see it, the RF is sharper but on that day, both lenses generated great images.

Swapping the 24-70 f/2.8 (EF Mk II vs RF). The biggest things that have caught my eye are sharpness at 50-70 mm (win for the RF), IS and weight. Again, I've seen real world comparisons, I believe the RF is just a hair shaper at those focal lengths. That would be nice. Regarding IS, looking at Bryan's IS tests with the RF and they are only a little (~1 stop) better than I get with my EF lens (IBIS). Avalanche Falls (1.3 sec) in the monthly was not shot with a full tripod. Those shots were either using a tripod as a monopod or resting on the guard rail. From Banff, I already posted a 0.5 sec handheld image. I took 3-4, all were sharp. Weight, the RF is ~100 g heavier. With converter, they are essentially the same weight.

So, from above, you mention that IS was a primary reason for switching. I do think IS+IBIS is better, but maybe only ~1 stop. If you still have your EF 24-70 II, you might want to try IBIS only (making sure it is on, Picture/camera tab 7, mine was off when I received it). Or maybe the sharpness at 50-70 mm is noticeable? I'd be interested if it is.

If you have both 100-500 and RF 24-70, then, so, you can start leaving the converter at home, which saves 130 g.