Arrive somewhere with good horses and poor leadership on the edge of the Roman Empire, implement Mongolian horsemanship/mobility/unit-tactics, most of which should be possible with the current day's technology in a horsemanship culture -

Is that true? Edward Luttwak in his book on Byzantine strategy remarks that the Mongol's reflex bows were very complex to make and hard to master - the Byzantine cataphracts had to constantly train just to keep their horse archery up to snuff.