what exhuast system would be best to imporve the performance of a 4cylinder?

theres alot of atermarket cat-back systems out by gibson, magnaflow, flowmaster etc. Guess you;d really have to check their websites and see readouts on the 4cyl exhaust they have and compare. I'm personally trying to find a dual rear exit exhaust, not that seems like it's gonna be impossible not to go custom with it.