Working at a BMW dealership I saw a lot of mufflers, and the majority of those were Boysen. I have sold a lot of mufflers in the after-market also, and when they were Boysen, they were the same as the dealer, because they were the OEM mufflers.
Boysen is an OE supplier to BMW, Volkswagen, Audi and Porsche and when available would be the OEM choice for the best muffler for your car.
Boysen develops and manufactures exhaust mufflers, pipes, catalysts, resonators, and complete exhaust systems for European automobiles. The main scope of their business lies traditionally with OEM, Original Equipment Manufacture. Some of their OEM customers in the European automotive industry include Audi, BMW, Porsche and VW as well as the English brand Rover.