Well, since most "mainstream" RFs of that period had fixed lenses, it makes sense that with the immense range of lenses available for the M-mount (including adapted LTM lenses) then sure you could take better photographs, because the FOV/framing/aperture you require for a certain shot might just not be possible with whatever FL the fixed-lens camera happens to have. While there were some outstanding SLR lenses of that period, they're not RFs, so they don't count.
The very bright VF and accurate RF might well translate into a higher proportion of keepers, and the ergonomics of the M6 TTL I had made me want to shoot with it in a way that I was not used to, therefore I got more opportunities for shots that would otherwise remain untaken.
But I'm not sure that's precisely what you are asking - do you mean that if you had a test scene set up with a tripod and swapped a Leica (presumably with a Leica lens) for a generic 70's RF and took identically exposed and framed shots, would you be able to tell them apart? Based on my own experience I would say yes, but perhaps not dramatically, even under such tightly controlled conditions (I have done this type of testing using the same roll of film and a mid-roll rewind for 2 cameras, to eliminate film and processing as variables). The pre-asph 50/2 Summicron is notable for its contrast wide-open, resolution right into the corners and lack of distortion. However... there are so many other variables (which include things like ergonomics and the ease/speed with which you can get a shot in "real life") that can affect the image that it's really hard to give an answer to the question I suspect you are asking, which might be better stated as "are Leica lenses noticeably better" (maybe I'm worng about that interpretation). Sometimes definitely, sometimes not so much - it also depends on what you're photographing. For example, field curvature could ruin a test-chart or landscape shot but work in your favour for a portrait or PJ-style image. And so on.
Apologies if this is not the sort of answer you were looking for, but I have found that like many deceptively simple questions, the most appropriate response is "It depends..."