I think you are overthinking it maybe. First in these parts (Europe) and I would presume in every metric system country there are two main rule lengths, namely 15cm and 30cm. 12'' is almost 30cm so the speed/distance equivalent in SI units could easily be 30cm/22.5cm/15cm/7.5cm. Multiplying those by 0.4 (or dividing by 2.5) you get the dungeon movement distances in meters, namely 12m/9m/6m/3m, and multiplying those by 3 (or by 1.2 if using the former tabletop cm metric) you get the wilderness (or non dungeon) movement distances in meters, namely 36m/27m/18m/9m.
For completeness at least in the distance department hexcrawling is a simple conversion from 5 miles hexes to 8 kilometers hexes.
As you rightly say the conversion to me (and probably to most metric system players) comes quite easily as we have been doing conversions such the above (or similar) when playing, so in the end it is not a big problem.
I doubt many metric system players play the game in imperial units as they are not very intuitive but I may be wrong in this point.