[Klaus]

This is what I believe too.

But, inevitably, this means lens X will perform very differently on different systems.

Because of the retrofocal design for wide-angle lenses, and perhaps less difference in sensor stack thickness, it wasn't much of an issue on DSLR cameras.

It may depend on the specific lens but generally, we haven't seen big differences with respect to cross-system (or cross camera) performance. 

Color casts etc are generally avoided with retrofocal designs (because of comparatively flat angles).
Symmetrical designs are more problematic (but rarely used these days).

FWIW, the Voigtlander 10/15mm are symmetrical.