The generalized thought that wide angle lenses would be better on RF bodies isn't necessarily as true as it once was. Back in the day, it was impossible to make aspheric elements and retro-focus designs suffered in comparison to the wide angle lenses on RF, especially the Biogon. Nowadays, you will actually see RF lenses that are using retro-focus designs in order to minimize vignetting issues, but they are top notch performers. My guess is that if you use top quality lenses on both and put them on a tripod, the differences will be minimal. That said, there are some Leica M designs that are probably the best that can be had for wide-open shooting, but I think that has to do more with the particular lens than a generic RF/SLR difference. There may still be a slight advantage for hand held work with RF for some people due to the absence of mirror slap...

Isaac