I am a newbie here too, but since no one answer your question, i'll try to help you with what i know. I have a custom catback 2.25 piping and a TRD muffler. Sounds good, maybe gain some HP. I would suggess you get something like HKS, Greddy, even something like OBX muffler w/2.5 inlet and 2.25 piping. This will give you a noticeable HP increase and a really deep tone. If you ever go for 2.5 catback piping, it will sound like a "fart" can and lose alot of torque as it is.
hope this helps