If you're replacing just the mufflers it will not really net you any real horsepower although it will give you a more aggresive sound. Your 99 exhaust isn't as restrictive from the factory as previous years. If you really want to open up your exhaust you'll want to look into an H or X pipe for better flow. As far as being a waste of money, it never hurts to give your ride a good deep growl so if you can get a good deal go for it.
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66 Tahoe Turquoise Coupe
Laser Red 96GT Ragtop
Both are slow but easy on the eyes!