pdm dump pipe

ok i know someone selling there pdm dump pipe to me for like 30 bucks.. i don't really know if i need it.. i have hotshot performance intake, and apex n-1 dual on the way.. is this dump pipe a good idea?.. what wil it do for my car, i've heard it frees up exhasut, butdoesn't a cat give u back pressure, and changes ur gas filled emissions into soemthing ready for the air.. well give me any suggestions.. so i dont; do somethign dumb.. thanx
-dave