I've been reading about people swapping the coil in their Subaru with the coil from a Neon R/T, since it supposedly gives a higher-voltage spark. Some of these people claim increased fuel efficiency, smoother idle, etc. Others report that they can't tell any difference at all. The suggestion is that those who noticed improvements had a worn-out coil to begin with, and that they would have noticed the same improvements if they had simply replaced their coil with a new OEM coil.

I can't seem to find much consensus on whether or not coils can "go bad;" some believe that as long as a coil still fires, it's fine, others think that they slowly wear out over time.

Can someone on Oppo explain this to me? Do coils wear out, and if so, what actually wears out?