A narrow (and pointed), jacketed, high velocity round will always penetrate better than a slow, broad, naked, lead sphere. That's why the wounds are "cleaner", they have a greater chance of leaving the body in the same condition they entered it with minimal deformation of the bullet and much of its kinetic energy retained. Exit wounds tend to be wider than entrance wounds due to the fact the bullet is pushing outward rather than inward, but a musket ball hits a broader surface area on contact, often slows to a complete stop in the body, and flattens itself or fragments into multiple pieces which diverge inside the victim.
As far as metal on metal goes, the 20th century cartridges will out-penetrate any muzzle loader, rifled or not. If a matchlock can reliably defeat your plate at a given distance, you should go nowhere near an MG 08/15.