Has anyone gone aftermarket with your exhaust system or muffler? If so, how did it sound? The motor doesn’t really make, uh, "sporting" sounds, if you will, but I say if it will help with power and MPG'*...why not? A local shop suggested pulling the system off of a Bonneville for a lil power and sound.
Leave teh exhaust on your Bonneville, that'* 2.5" stainless. You can swap out the muffler for a series80 (singel) or series40 (dual) for a louder deeper note. A free flow cat will deepen it more, different resonators will also change the tone. But leave the pipe as it is.
Your not going to benefit or see and gains from switching the entire exhaust system from another car. A high flow catalytic converter and different mufflers MIGHT give you a very small increase in MPG'*.
Your best bet is just changing out the muffler(*) to get some better sound. I have Thrush Turbo Mufflers and it made the exhaust just slightly louder and gave it a deeper sound.
If you want a MPG or performance increase, your best bet is to focus on your intake, rather than your exhaust. You can change out your air intake for a FWI (Fender Well Intake) or CAI (Cold Air Intake)