It's an additional 10% piercing and 15% slash protection but the design philosophy is no different to player armor. It's intentionally, by design, supposed to have minimal difference between armor tiers. This inherently puts heavier value on player skill, not gear quality. The only way they buff horse armor further is to increase the penalty on speed, turn and stam further - which I think needs to happen irrespective of buffing horse durability. Currently there's no point in taking anything but heavy horse armor.