Never a good thing to have strange bits bouncing around in the cylinder. If you are deciding to go to expense of pulling head, maybe find a shop that has the scope to look into the cylinder and examine the walls/valves/pistons first. I replaced the plugs on my 560 recently and part of the tip insulator was MIA. Plug looked like the devil so I don't know if I had a wire failing or just spark going every which way with missing insulator area. Compression seemed ok on all cylinders, though not like new (345k miles). I believe detonation can cause damage to insulator.