I'm curious about this as well, although I have no complaints about the performance of the five Contax lenses I presently own (35, 45, 55, 80, 140).
That said, I can say the two 45-90 zooms I have had performed similarly. Unfortunately, they were merely good performers, not great, and comments I have read/heard from others suggest that there are indeed some great ones out there, which is what has started me wondering.
In theory, there has to be some amount of variability among them, but the extent to which this is visible is another matter. I look forward to reading about other people's experience with them, as I have been tempted to start buying used ones to compare and keeping the best of them because I am planning to continue using this system for many years to come.