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Leo's cat is not "high flow", but I suspect its better flowing than the factory cat. Dual exhaust cars like the S4 use a pair of different style cats, different EO# with a canister shape.
All 928's react well to a more open exhaust, just making it a bit louder makes some owners happy.
Smog checks and what cats muffler places will install are still a bit up in the air. Even if I could get someplace to install it, I would not have a cat put on with an 09 date code unless the CARB EO# matched up to my car specifically. Investing in a couple old stock (pre 09 date code) cats might be a smart plan for the future, but I haven't done it yet.
I need cats good enough to fix any emissions issues, and I think that excludes the highest flowing cats like the Random Technology stuff. I also don't want anything drawing smog attention to my car, so the more legal the cats are the better I like it.
X or Y merge pipe, better flowing cats and muffler will enhance whatever the motor could do in factory trim, so gains for a 345 hp GTS will be a lot more than from a 234 hp early car, but all three on an S4 and 25 total rwhp gain seems typical.
Given the car you are looking at, I say LEAVE IT ALONE, drive it a few months before changing anything.
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