OK INTRUDER....let me see if i got your last correctly.........
A normal guy in an A340-600 has better vision than another normal guy in a B727-100 simply because he's flying a newer aircraft?
Dunno where you got THAT from -- certainly not from me!
Either one of those airplanes/crews may or may not be Cat 2 or Cat 3 certified, and either one may or may not be operating on an airport that has operational SMGCS.
The realistic taxi minimums for either airplane/crew will depend on the certifications, airport, and current weather conditions. As I REALLY said,
There is no "unified value" for taxi visibility that I know of.