I know I'm not demon but you might want an additional opinion. The stock precats aren't that restrictive. They use essentially the same precats on the escape with very little detriment in power. That is if the precats are flowing like new.
If they are plugged then it would be #1 in the list.
List:
Stock crush flange in the Y-pipe.
Stock panel airbox.
Stock resonator.
***Rear 1->2 y-pipe on quasi-dual SVT system (sometimes found to have metal restricting flow and necking down)
Stock muffler.
***Precats***
Stock UIM
Stock TB.
Stock heads.
Stock LIM.
