With all due respect, the speed it already has does a hell of a lot of damage when bumping enemies.
I recieved 50% (18 Str no IF) damage from a full speed courser (probably loomed) while I recieved only 30% from a full speed plated charger bump (unloomed confirmed).
Granted the "looming" difference should be taken into account, but if I take less damage from someone with 6 riding on a plated charger, to someone on a courser, it's already half decent. Survivability should be lower in my opinion because of the fact that it's got the speed. (Logically thinking, less armour = less weight = faster running = higher speed bonus on charge damage.)
I am fully aware that these two could have been a fluke, and I have tested and found that the stats are roughly the same each time, but I can't see any reason to give a horse more speed, and therefore more damage, or increase it's survivability when it's a massive bullet already!
My opinion naturally, being a lowly foot soldier!