just make sure you get the resonated exhaust. without resonators its way too loud. if u want to keep it even quieter then keep the cats but performance wise those primary cats really gotta go. I went with full jhm with their high flow cats and resonators and I absolutely loved the sound. the jhm lightweight crank pulley is also a really good mod for this motor. jhm intake spacers help keep heat soak out of your intake too and have been proven many times.
jhm exhaust
jhm tune
jhm crank pulley
jhm intake spacers.
that’s a very solid na rs4 that will run mid 12s all day long.