I have been wondering about this as well- I may be thinking of the wrong cat but when I asked my muffler shop friend he told me that there wouldn't be any noticeable gains from this...just more sound.

99 SE Sport 2.5 Duratec
Spruce Green Metallic..
Aesthetic Only.... For Now

...the closest I will ever get to having a Turbo will be the Gillete Mach 3 variety!!!