That 3 MW throwing lances from horseback example is a bug, currently xbow/throwing are both unintentionally capped at 15 damage from horseback, which will be addressed next hotfix/patch.
I'm not certain that the 20% damage nerf on horseback is intended to affect xbow/throwing either.
Until then, if you're going to be mounted ranged, aim for horses or use low-ish damage builds so that you're not affected by the cap.