If you plan on racing the car and want high end power in the upper RPM ranges, an aftermarket exhaust can help, at the expense of low to mid-range power/torque, throttle response, fuel economy and every day drivability.
Some people don't care about that though and just want, "that sound", even if it means the car is slower and the fuel economy is decreased. Different strokes for different folks I guess. :icon_confused: