I agree with giving a small hitpoint/armor buff to the horses. The thing is the tier 2 and 3 horses have a rather high level of both maneuverability and speed (especially courser and arabian warhorse). Part of the original problem was lowering agility requirement to 3 per riding skill, so raising the riding requirement is not going to fix the problem as most cav have over the requirement already because you can still do a balanced build and get 6-7 riding and each point in riding skill significantly increases the horse's speed and maneuverability.
Prior to January patch throwers were not OP because most archers countered them because the archers were OP. After the archer nerfs in January, it finally became apparent over the next few months that throwers were OP (by the way the thrower nerf was a trifle too harsh, coming from a guy who hates throwing). So too OP throwers and everybody being able to be a pocket pikemen kept cavalry's basic OPness in check. But now with most pikes gone except for a few dedicated pikemen with no other weapon and throwers nerfed badly and hybrid xbowmen/archers significantly reduced in amount of ammo, cavalry are having a field day of OPness. I am hoping it doesn't take another 3 months, like they took with throwing, before balancing cavalry, Ia lso hope they don't go too overboard like they did with throwers.
My suggestion: Make riding a 6 agility skill requirement again and reduce the riding requirement of all horses by 1 (I would love to see mounted infantry again with sumpter horse riders). The few people who still want to ride the plated charger can, just as almost pure agility, but most people will have to do balanced builds and have 3-4 riding skill. This can be accompanied by +20 hitpoints, +5 armor on all the unarmored horses and +2 speed, +2 maneuverability on all the armored horses. We would see cav diversity on the battlefield again and there would be real tradeoffs for choosing riding skill as it would require a stronger commitment to agilty, so less hitting power and hitpoints.