For me, I already have the headers, and a Borla cat-back.  There are two, 2" pipes that come off of your exhuast manifold.  It then flows into one, 2" pipe and then back into two seperate 2" cats.  I plan to install a 3" Magnaflow cat.  I will eliminate the choke point of the single 2" pipe and will be able to install a bun for the 02 sensor as not to throw the CEL. Theoretically it should give more power, and the total operation is less than $250.  The shop say if I don't like it it will be about $30 to change it back.