Most definitely a defective/non-functional catalytic converter will attribute to some power loss.
With respect to the AP1/2, the OE catalytic converter is already very good in terms of flow and isn't required to be replaced by a more substandard local made product when changing a catback exhaust. (DC2R owners would select the OE AP1/2 cat as an ideal upgrade just like their JDM exhaust manifold).
To extract maximum gains in this department, it'd be optimal to run a test pipe matched with a N1 single output catback. I'm sure you already know that.
in my friends commordore the cat was stuffed
he then pgraded to a high flow cat instead of standard and noticed better performance and fule consumption
sounded better as well
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