I seem to remember this being asked already in another thread..
edit - I'm not crazy http://www.perth-wrx.com/vb/mechanic...ew-prices.html

Hitech or xforce seem to be go-to options.
I have a butler system with single resonator and oval muffler and it is quite good.
Xforce even do a varex muffler than can be opened and closed when noise is wanted.