I wonder if the dynamic range might suffer though. Four times the resolution sounds nice, but that also means that the signal has to be amplified by four times and the noise with it. Of course noise gets lower if you average four pixels, but that does not work with a lower frequency noise. Also the R5 has some issues with colour shift to green or magenta when you want to recover shadows at high ISO. That does not happen with the R3. Is that just because the R3 has a stacked BSI sensor?
The upside of having four times as many pixels might be that demosaicing the RAW file might work better, as you have one red, one blue and two green pixels for every pixel of the low megapixel result. At the moment camera manufacturers cheat with the megapixel count, as they add all the coloured megapixels separately