yes cat back means after cats...you would have to cut off the flange and bolt it up....yes it would be a little louder...but if your going to go that far, i would recommend either a better h pipe or x pipe (mid pipe) and then the rest of the cat back stuff.
For now, go with what you have...it will be fine. Here is my list of things you should do next....
upgrade the procharger idler pulley to a reichard racing pulley (a must, that one they provide is garbage)
suspension...not just springs...subframe connectors, k member, control arms, shocks struts...all the rest will depend on if you want to run on the strip or road course.
forged internals
long tube headers and supporting exhaust
upgrade to d1sc
cobra tank upgrade
up the boost
probably forgetting stuff...but hey modding is never done till you no longer have the car.