Did anyone ever wonder why they had the muffler from a 330i? The M54 engines got 3 mufflers/resonators and 2 cats in the exhaust manifolds. The M56 got 2 mufflers/resonators and 4 cats, 2 in the manifolds, 2 in the front section of the exhaust. The 330i muffler was used because it had the active exhaust butterfly valve and therefore could manage the noisier exhaust note. That explains why the M56 after doing the 'golf tee mod' tends to have a much lower exhaust note compared to other cars.
This would also explain why BMW does not market the BMW Performance exhaust for the M56 cars.