We had a twin turbo GTO at the shop that had a stock catback. The car sounded really quiet and tame, you would never believe that it put down well over 600 rwhp! With the cut-outs open it put down over 700 and sounded like a complete different animal. The factory cat-back was restricting a little over 50 hp.
While tuning the car I did not notice any weird hissing sound, it literally sounded like a car that only had a mild cam. Probably one of my favorite sleepers.

 

What happened to the vid I posted??

 

50rwhp!?!? Geeze, might need to throw a cutout on mine, I had no idea it was holding back that much power.