The diopter adjustment was fine. Brightness/contrast maybe a little more "contrasty" than the R, but I'd have to A/B compare. Could have been the store lighting. Brightness was ok for the store at least. Had a LOT of trouble seeing the corners. With the 5DmkIII I can see the whole screen with my glasses on. This was very much not the case with the RP. With the RP, it was like my eye had to dance around under the shower to get wet (visually speaking). I'd be using the EVF better than half the time, as I tend to shoot in full daylight.
Canon once made a do-hickey for the 5DmkII that changed the viewfinder image, allowing a smaller but entire screen to be seen at once, and I've got one. Works ok, pretty well in fact. But, that won't do with the RP, as the detail on the screen is pretty small to begin with. I'm thinking I remember seeing an adjustment for on screen display sizes from reading the manual, but I could be confused on that. Either way, Canon doesn't offer that type of eyepiece correction for the RP (at least not right now).
There are other considerations too. Camera didn't "feel" all that great, but I could get used to that. Seemed too light... sort of toyish. I know the target market is those folks "moving up" to full frame, fully admit from shooting 5D series I'm used to having some heft in a camera. Like I said, that would grow to be something accustomed to.
The main reason I considered the RP was the smaller size. Even though the RF lenses seem very big, the bodies are not all that small actually. A little smaller than a 7D series, a lot bigger than an old Rebel XTi. Of course, those are crop sensor cameras too.
My hope was to be a bit more stealthy, with an 40/2.8 pancake and adapter (which is smaller, together, than the RF 35/1.8). I don't see being any significantly smaller with the RP over the R, and the R has features I really like - like the EVF is pretty nice, plus shutter over the sensor, same battery as my 5D series cameras (a plus since I travel a bit, don't need extra crap). All the rest is just vanilla vs chocolate to me, and I don't shoot all that much video - only very very occasionally some ten or fifteen second clips like old newsreel stuff.