The cavs in the game do not act as shock troops or flankers - the vast majority of their kills are against unaware opponents.
The problem with the horses speed is that it turns them into assassins: the distance that you hear the thump thump of the approaching horse is crossed so fast that there is no time to turn, locate them, see if they have a friendly icon and move/block/attack. They are effectively silent. Maneuverability gives no defense against arrows - what the 9 G turns do is to allow to break off an assassination attack against an aware target, so there is no commitment to the charge.
The afterburner acceleration allows cavs to bump into objects at full speed (no collision damage) and immediately start moving again. This combined with the ridiculous turning rate permits cav to ride at full speed in narrow streets with fear of killing themselves or simply being stopped. They are also not slowed down much when bowling through a group of people even with a low charge value. It also allows to circle-lance at 0 speed.
Speed is the most important stat that needs to be lowered across the board. A little more HP (not armor) to the squishy horses is acceptable.
