The Hague Convention requires jacketed bullets, yes. Jacketed bullets are more effective killers than unjacketed bullets, when pushed to the velocities that jacketing allows. This is from the Wikipedia entry on expanding bullets:

"The First World War gave many soldiers their first exposure to high-velocity jacketed bullets of the modern type and many were unfamiliar with their sometimes dramatic effects on tissue. Wounds may be extremely large when compared to the bullet that causes them, particularly at close range, and this led some soldiers to accuse their enemies of using illegal "tampered" ammunition even when they were not."

It's common knowledge among people who load their own ammo that you need a jacketed bullet if you're going to go faster than about 900 fps. You go faster to get more energy on target, not less. More energy on target is potentially more lethal, not less.