For everyone considering an after market exhaust I recommend purchasing what ever kit is designed for your car.  Most people that buy cat-backs or full exhaust systems have relatively stock motors.  A lot of people believe that having no back pressure is that best thing for the engine in order to make power.  This is a misconception because only horsepower is on their mind.  Keep in mind that horsepower is a bi-product of torque.  Bigger is not always better!  With too big of a pipe diameter your motor will lose a significant amount of bottom end power killing your off the line performance.  Exhaust companies such as Magnaflow and  Flowmaster pick a pipe diameter relative to the amount of air your engine can flow and increasing air velocity potential.  An engine is nothing more than a glorified air pump so having the correct pipe diameter will actually help increase the air velocity of air flowing through your exhaust.  The more air you can flow through the motor the more power you can make.  A proper exhaust system can actually flow better than just a straight pipe due to better exhaust scavenging (basically means moving all those exhaust gases out of the combustion chamber), resulting in more power.  Most V8 kits only go to about 3 inches in diameter because those motors make around 300 HP in most modern applications.  If they make more that pipe diameter is still ok.  So motors that are 3.8 litres in displacement or 231 cubic inches don't need to be that large.  Most exhaust kits are 2.5" or smaller.  True a larger diameter might affect the "sound" of your vehicle but I have yet to see a 3inch make more number on the dyno over a 2.5inch system on a stock motor.  So if you're a performance enthusiast take all the guess work out of "how big of an exhaust should I go with?" and trust the companies that invest millions into this area and go with what they say.