a full exhaust system will generally consist of headers(optional), downpipe, which is from the header down, the mid pipe after, and then the cat and cat back. the catback bolts directly onto the cat and is usually mandrel bent. this means that the piping diameter remains the same, regardless of bends. factory bends are usually crush bent, meanning smaller diameters at the bends.
different mufflers have different sounds. generally, the bigger mufflers will be quieter. however, companies like APEX makes some mufflers with a loud sound. i have the N1 turbo and the thing is so loud, it rattles my interior.
it is probably the best to get a cat back system, rather than a universal (just the muffler). usually, the catbacks are designed to increase horsepower, whereas the universal do, but since you are using the factory piping, it may not be as benificial
hope this helps
