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.
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03 GT, STB, GM HD sway bars, 35% tint, GE893(37watt)fogs, Flowtech Terminators, 180 T-stat, ZZP DP & PCM, Ceramic PLOG & Crossover, 18" glasspack, 2007 17" Mustang Bullits 235/55/17 Pirelli PZero Nero, 12" R1 D\S rotors, gutted air box, GTP radiator