The Magic cat is a good brand and try to aim for atleast 3inch or bigger as a minimun.
Also, get a split dump and front pipe - So the seperate wastegate pipe joins down nea the cat, this helps reduce any turbulence up the dump pipe end.
I have a trust system with two oval mufflers and it is very very quiet. So if you want silence go for two mufflers that are oval, but if you want it loud, go a single cannon.
have fun