Here is a vid of a modified GT-S with a Bertocchi cat and resonator upgrade from Bertocchi which is an exhaust from Japan, last time i drove a GT-S i noticed the sound at higher revs to be a bit quiet compared to how great it sounds at idle and low revs so i think this did the trick.

http://www.youtube.com/watch?v=5fI9f_p0GHg

 

awesome sound!