RE: Any more killing power with rifled barrel
The answer to your question is, GENERALLY, NO! The ammo manufacturers have designed their slugs for rifled barrels so that they have about the same absolute maximum range as those used in smoothbores. The reason for this is so that they will be permitted to be used in the same palces that the smootbore slugs are legal. There may be some excweptions to this, but it is generally true.
Of course, the rufled-barrel type slugs give more accuracy, and consequently are more effective for hunting purposes than the smoothbore type. But they won't travel any farther if fired up at a 30 degree elevation. Both will fall to earth at about the same distance from the muzzle.