I think a mobility mod should drop the penalty by a fixed amount - e.g, a green armour should only require one to be non-encumbering, whereas a Red Armour may require four.
The dodge thing to which you've alluded sounds interesting - but rather than a further penalty, you should make it more of a neutral thing. The game's difficult enough, after all.
E.g., blue armour is 'normal', red armour is +10% monster to-hit chance, green armour is -10% monster to-hit chance, no armour is -20% monster to-hit chance. This would also serve to make the initial level less frustrating on higher difficulty levels (when lucky hits from Former foos cause you to bleed down to 40% HP and you have to restart).
Also, I think your penalties are a bit high. Perhaps no penalty for green armour (its positive effect is paltry, to be honest), then 10% for blue, then 20% for red. And maybe even +10% movement speed when unarmoured (again, making that first level better, because you've got a decent chance to get the medikits and not have to restart in frustration).