I have a 95 SSEI and a couple of years ago I replaced the muffler. NAPA did not have a stock one for my year and could not get one. They said they were were discontinued. They had a a stock muffler for a 94 Bonneville non supercharged. The upside was that it was a stainless steel muffler and as such would probably outlast the car. The muffler shop had to do a slight modification to the exhaust connector to put it on. I noticed right away that it had a lower rumble than the original muffler. So it seems that the original muffler for the supercharged cars help to make them quieter. I am no muffler expert but this was true in my case. I suppose you could ask if the muffler that was installed on your car was specifically for a supercharged Bonnie. You could also verify through a dealer if the mufflers are different . At any rate I do like the sound and I have seen no reduction in performance.
1995 Bonneville SSEI
1994 Buick Park Avenue