Custom piping is good as well as its bad, unless you get the correct material to make out a exhaust pipe, your car is gonna 'HOWL" and the heat is gonna go up under neath which may damage other components and you will gain zero results.
Also newer cars are very complicated to work on, like having sensors here and there.
Custom piping is good for higher performance cars that have a huge potential (turbo charged,super charged) higher displacement monsters!
Its better to stick with the mods that other posted, and you will gain and feel a big difference in doing so.
And if you really wanna upgrade your exhaust then buy a full cat back exhaust systems with down piping, high flow cat etc made by a well known manufacturer.