From what I've read, no one here has failed an emissions test because of a Magnaflow. A friend of mine runs one, and though we don't require emissions testing in IN, I figured I might let you know. Anyway, the mechanic can go poop on a duck because the 3800 was designed with emissions first. One of the easiest ways to get this knowledge is here, and probably 4th or so in line is Wikipedia, which tells you that the original design was sold because it wasn't needed and was then bought back during the oil crisis.
You have the second latest 3800 design, and few thought Bonnevilles would run quarter miles, get new cams, and so on... at least in the design process. Though your emissions test might get a little bit closer to the NO side, it'll be far, far below other cars, even with the Magnaflow high flowing design.
If your car is running rich, though, or you know why the catalytic failed in the first place, please take care of that problem as well. That should help not only emissions, but the longevity of this new cat and the engine in general. If you get a used Magnaflow cat, know that it'* illegal for an exhaust place to install it, though you can...
Well, good luck, and you'll probably get lots of replies about the ones people here are running. My last bit of advice is to stick with your exhaust pipe diameter.