How does a catback give better performance?

i bought a 93 accord with a pasesetter catback an i dont like the sound, its a 2.25 piping. i want to replace the muffler with a magnaflow 14829 muffler an take out the cat. what i am wondering is thins catback doing anything or does it all lay in the mufflers hands? also if so whats the best muffler without getting a fatcan for sound an performance?