The near-universal recommendation is that the C55 is too quiet from the factory and that deleting the resonator (and often the secondary cats) unlocks the V8's voice without becoming obnoxious. Commenters note the secondary cats have no oxygen sensors, so removal won't trip a check-engine light, and the primary cats keep the car emissions-legal.
Resonator and secondary-cat delete: the go-to exhaust mod
Mercedes-Benz C32/C55 AMG (W203) · mechanical