There are some legacy lenses that perform very well, especially stopped down. We see it with many types of lenses - Mamiya is not immune to this.
The biggest difference I have seen between, for example the 80mm lenses, is that the Phase One D and Schneider LS versions (BR and non BR) perform better wide open and also exhibit less chromatic aberration than the legacy AF version. Stopped down, the differences are much harder to discern. I found the Phase One 80mm D and Schneider 80mm LS/LS BR lenses to be virtually identical. They may share the same optical design. Anyone who states they have encountered variations - I don't doubt this, it is entirely possible. It is rare that I have seen significant variation in new lenses - especially Blue Ring - on the level where there is a clear advantage for one lens over the other.
More commonly I simply see differences - across the frame, if broken into 20 different quadrants, the middle upper right on one lens is slightly weaker than another, but the lower left corner is slightly stronger, with the end result being that the lenses are roughly equivalent, just different. It is easiest to notice this when comparing many different lenses, but especially many versions of the same lens.
But to be sure, I feel it is possible to get a "bad day" lens from time to time from anyone, regardless of the price. In fact, the more expensive and more hands on, then perhaps the more likely, as human intervention increases the potential for error (but also provides positive contributes to the process as well).
I have not experienced perfect uniformity with any lens that we've ever received, and this includes all of the legendary lenses you can think of, Rodenstock, Schneider, Zeiss, Leica, etc. If it happens, it seems extremely rare.
Steve Hendrix/CI