I've long thought heavy armour on both horses and infantry could apply a 1" penalty, and dwarves would get a special rule to keep their 5" movement. That's as far as I'd take it though, I think your increments are too fine and too much to track in a game.
Also, just a minor quibble, but the game was designed for inches. I know it's a metric world (I'm in Canada after all), but the game's spacial relations seem more balanced and work better with inches. We tried with cms for a while and the game was different (and better) when we converted because the cm conversions are pretty poor and inaccurate.
I wish GW had avoided the whole inch/cm thing and just called them "movement units". But then they'd try to sell special measuring tapes at $20 a pop
