I'm putting on a new exhaust system on my 2004 Grand Prix GT2. I'm going to have a 3" downpipe with a high flow cat converter and I'm thinking of putting on two Flowmaster Super 40 mufflers. Has anyone tried these on a Grand Prix? I've heard and read nothing but good things about them.

I have them on my GTO. While its not a GP I am very stasfied with the nice deep tone they have. Just keep in mind that they will get louder when/if you do any mods to the engine/intake

Personally I believe flowmaster's are over rated. They are too loud IMO. All they do is announce your presense to Johnny Law and annoy your neighbors when you come in and out in the wee hours of the morning or night. Not to mention that drone inside your car.
I just installed Magnaflows ($66) on my wifes 04 Monte SS. They sound great. I then did Summit brand fully welded mufflers which were $38 each. The two cars sound almost identical.

I have magnaflows on my 2004 grand prix gt2 and i love the way it sounds. Out of all the mufflers i've heard on a grand prix the magnaflows is my favorite. There not to loud but you can hear them. They just have a good deep melo sound.