I had done this exercise: Side to side shots with 100 primes: FD100 2.0, EF100 2.8M II, RF100 2.8M, Milvus 100 2.0M, all with the R5:
Yes you see the difference! This is not about pixel peeping, this is all about colors and rendering: With every composition you can tell the Zeiss by the most pleasing set of colors (and you can tell the FD by a very pleasant less saturated color rendition as well
Comparing the Milvus 135 against the RF 135 the Zeiss rendition is the better in many aspects. But the RF owns AF and IS and collects a third of a stop more light, which is somewhat handy from time to time.
The OTUS ML85 might become a brother of the EF85 im my arsenal, I was never interested in the RF85.
Color rendition is something quite subjective, and depends on many factors. Monitor, exposure, color temperature etc...
When for instance comparing Leica M and Zeiss ZM lenses, I tend to prefer Leica colors, less saturated. Yet, I nevertheless like the Zeiss ZM colors very much...But this can change from lens to lens, some ZM are more, some are less saturated . And it also depends on which software you use, LR or else.
Zeiss EF lenses have -for me- very pleasant colors, but nicer than Canon's EFs? Hard to say.I wouldn't go so far.
I do believe what you wrote, but would I see the same advantage for Zeiss? Possibly, but not certain.
One fact is a certainty: Zeiss produce some of the very best lenses around. But, just like my Leica R lenses, I don't use them wide open, unless for static motives. Lack of AF is for me, a serious drawback.
PS: To be clear, I use only Zeiss ZM and Classic, neither Otus nor Milvus. If I did, my statement could differ.
But maybe, the next Otus will be AF? Then I might very tempted...