As @
churchx said above, it is not about the cat, it is about the long tube headers / manifold that gives the performance gains and eliminates torque dip. Making a catted long tube header is expensive and hence most people prefer the decatted one. You can buy a catted one from FTSpeed, if you like and can spend the money.
Same hardware aka OE header will not give you any further gains as its short tube.
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