Usually in terms of gain for us, a hi-flow cat is most of it. The muffler is only 1-3 hp, unless like willwren said you are making prodigal amounts of power (400 WHP). The sound is the main issue of course. Also a hi-flow muffler may get you better gas mileage.

The best thing would be a tru CAI, Open up your headers, Highflow Cat/res combo, with your 80 series muffler. With this done you car will be breathing and passing exhaust gas better.