As long as a horse has some armor (even cloth), it's the same as every other barded horse.
Are you sure this includes their moving speed ?
From my observations, Fast pony > pony > heavy horse > other mounts (i havent checked eastern armored, barded etc against each other).
Besides, the mass of a mounted unit is directly linked to the unit itself, not to the mount, or is there something i missed ?
What file contains the mount datas ?