I'll throw my .02 in on this. I think that you should go with the quietest muffler that you can, maybe even use 2 mufflers in series. You can always install a dump pipe if you need that extra couple of tenths at the track.
I've had both quiet and loud cars. Here's my reasoning:
1. There is minimal performance gain from a louder, less restrictive muffler. This is from personal experience, but I wish that someone would try several different mufflers at the track (not on a dyno!)
2. Quiet cars do not attract attention from the reckless driving ticket writers; a.k.a. the Police. Think about it, when you hear a loud exhaust, your head turns and you wonder if maybe they want to race. We all love to race our Buicks. Just admit it!
So, with a quiet exhaust system you get 98-99% of the performance, and you can have more fun because you can get into the boost more often because of the lack of attention.
Side note: My car is super quiet right now, and I love it when I take it to the track and line up next to some big-block muscle car and then have to look at my tach to make sure that my car has not stalled at the line. Then, when I cross through the traps first, I then wonder if I've blown a head gasket because I can't tell if my motor is running because the open exhaust and big cam of the other car makes it impossible to hear mine!
The most fun I have with my GN is always the look of surprise in defeat by other drivers who are not familiar with our cars. My quiet exhaust inevitably causes the other driver to underestimate my car. My car may not be the fastest GN, but my simple combo has been loads of fun for me and loads of disappointment for a certain Viper owner in my area.
The most important thing: Have FUN and enjoy your car the way that you like it. That's really what owning a Turbo Buick is all about.
Mark