Installing an aftermarket, high performance sports exhaust system are a great way of enhancing the performance of your car. Besides that can also be used for cosmetic purpose as well as to produce a beloved roaring and growling car sound which makes your car prominent and more prone of being noted by your friends.
First of all I want to tell what an "aftermarket" means for car parts and accessories. Aftermarket part means that the part has not been and factory fitted or it has been release after the release of the car's model. It may also mean that the certain part has been manufactured by any other manufacturer.
The exhaust system acts just like a respiratory system of a man where the different gases produced by the working of the engine are provided a passage way or exit route through the exhaust tubing. When a man works out, the lungs helps in expelling out the carbon dioxide from the body and provides a departure platform. If the gas does not exit from the body we will suffocate and you know better what happens next.
So in order for an engine to work more efficiently, the best thing would be that exhaust piping are of larger diameter so gases exit more rapidly and there is less restriction for the gases to pass out. This benefit is what we gain when we replace a factory fitted stock exhaust system with an aftermarket sports exhaust system. Since they provide relatively a larger diameter piping, there is a less pressure of exhaust fumes which results in higher horse power but on the other hand a larger bore size of the pipe slows down the movement of the gases. So performance gain does not lies with fitting the biggest possible bore size pipe as it may result in the loss of low end torque, but an optimum size is suggested which definitely varies with the type of engine according to the RPM produced. When a standard exhaust system is replaced with a new high performance sports exhaust system, generally 10-15% increase in horse power has been observed.
Another problem with the normal exhaust system is that they manufacture a crush bent exhaust tubing which means that there are spiral rings around the exhaust piping due to which the diameter remains uneven and restrictions for gases to pass through. On the other hand high performance exhaust system generally provide mandrel bent exhaust tubing where the diameter remains same all over the system.
So making the debate short, while replacing your standard one with a high performance sports cat back exhaust system, you should be considering the size of bore of exhaust tubing related to your engine specifications. Similarly one general tip of making the gases to exit with a greater velocity is that the the big pipes are connected with small pipes along the entire exhaust manifold are connected in a cone shape. This of course has some demerits too. Similarly if your exhaust system creates an unwanted sound, you can use aftermarket mufflers to kill it.