Exhaust, Mitsu lancer es 09 w/ cvt?

[stopthebreaks]

I hear an exhaust helps improve performance but how? the stock one and then ones ive been looking at like greddy ti-C look the same except the hole is wider. If the air outflow helps, why didnt the manufacturers create a bigger widening?

If it helps performance and makes it sound nicer? what exhaust should i be looking at? I want something that sounds nice but not too loud like the Ti-c and spectrum.