I mean they either spark or they don't. If they spark then there's usually no difference in how the car runs. If the new plugs don't spark it's gonna be a fun time chasing electrical gremlins.
carbon buildup is mostly soot, glued together with oil. no soot=very little buildup. if you take the intake off of a pentastar with no egr, you will find it pretty clean except for a sticky oily film in the runners.
You would be surprised how often this works, especially on diesels with variable geometry turbos.
Cars need to be revved out once in a while, and on the diesels the turbo vanes can get stuck if you never make them move