And your bottom line is what really matters in the end.
From my own experience I can tell that wide open my Summitar and my Pentax lenses are different in a noticeable way. The Leica vignettes more and yields a type of distortion that doesn't look very good especially with stray direct light in the scene. The Summicron I have tried is much more neutral at its widest aperture, very similar to the Pentax.
Stopped down to 5.6 or 8, which is where I like to be for most things, I literally must check my notes.
I don't print very big, only to about 16x20, using a Leitz enlarger. If I printed bigger, I couldn't tell you how they would fare.
But back to the bottom line, the best photographs are had with those cameras we love and are used to using. The small differences in lens quality are highly insignificant if you think about the total importance of any photograph. Would iconic photographs be less iconic if they were made with Pentax or Olympus lenses? I don't think so. Who cares?