I've found the the 85L f/1.2L II focused well on the 5DIII and better on the 5DIV. With both bodies, the 85L II and the 85 f/1.4L focused better than the 50L with the outer AF points. I was tempted when there was a Canonpricewatch deal for the R, but I'm looking for something at or above the 5D level.My 85mm 1.2L II focused significantly better on points outside the center (though not at the very edges) on the 5DIV than it did on the 5DIII; however, I still wanted snappier AF plus IS, so I traded it in for the 1.4L IS. I think it was the right move.
I would have thought the 50mm 1.2L would work better on the 5DIV also. Sorry to hear that isn't the case for you. Please follow up if you do try it on an R body!
While these lenses are welcome updates of Canon's highest end EF lenses, they were likely in the pipeline for some time. I also expect that some of the updates help these lenses work better with the RF adapter, as Canon has already said that lenses released before 2012 will not take advantage of all available RF features.What about the 400mm F2.8L iii and the 600mm F4L that were announced earlier this month? Don't they count?