I don't have a Mamiya rangefinder, nor have I done head-to-head comparisons between my Leica M and my Hassy (which essentially becomes a 645 when cropped), but there's a point at which the extra real estate of the larger negative has to trump even the best Leica lenses. That's simply the physics of the film coming into play.
I frequently make 11x14 prints from my Leica M images, and they almost always hold up well. But, I shoot mostly FP4+ or Provia in the M, except for night shots, so the negatives would be expected to hold up well. I have not, however, tried making 16x20s from the M negs, as I don't really like overly-large prints.
Given the difference in design and media characteristics, however, I'm not sure it's really productive to make narrowly-based comparisons of this nature. Each camera/format has its own sweet spots, and should be used accordingly, I feel.