I voted no.
We seriously don't need a buff for HA and HX, and since the horse don't even slowed down when hit by a sideswing from a poleaxe, i don't see why it should when hit by a tiny little arrow. It's all for balance. Speed is very important for cav, and lowering it (when it's already not that difficult to kill the horses) would be too big of a nerf.
Need to "change" them in other ways.
*cough*remove couchable tag from all but Great lance*cough*
*cough*bring back the old lance angle and bring back the old speed : so -3*cough*
*big cough*reduce the length of all thrusts animations from horseback by 10*big cough*
If they want more speed with less hit speed reduction then they need to choose a horse with armor. Its called balance. No armor and fast, but penalized by arrows, or more armor, slower, less penalized.