Delete your resonator, and put on a Megan or Ansa or Magnaflow, and you will be smiling. And, to answer your initial question, the guy that told you that was full of ****.

Oh, and if you really want to get even louder, you can remove your back 2 cats. Only the front 2 cats have the O2 sensors in them.

By the way, the resonator and OEM muffler are extremely heavy. Removing them will lighten the car thus increasing performance.