There are a number of opinions as to which is the most effective exhaust mod to make to the C2/4's. From replacement of the complete system with an aftermarket header and muffler to replacement of either or both the primary and secondary muffler. You choice will depend on money and desired effect.
For me, this is not a performance mod for racing - I was looking to free up the exhaust path and provide a nice sounding exhaust without irritating the neighbors. You might call this an enthusiast's modification.
I choose to replace the primary muffler (the one across the rear of the car under the bumper) and leave the Cat and secondary muffler in place.