1st thing to start is engine oil lol....
the factory catback is actually very free flowing, the choke points are intake/header/cat.
Your heavy 19inch wheel probably don't help either, since 1kg of rotating mass = 6-10kg static mass... 19 inch wheels are usually 10kg each or more, thats 2kg per wheel more than the stock 16x6 rims x 6-10kg rotating to static mass conversion....
it's like an extra 70-100 odd kgs the car has to pull...
OEM cat pre facelift has 900 cells... go figure
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