It uses Magnflow resonators. BW also offers a non-resonated version. Both systems are made in the USA, and Magnaflow uses 100% US stainless steel, but depending on component is either 304 or 404 stainless. 304 is the high polish steel you see on the resonators, but 404 is reportedly easier to work with and a little more durable, which is in the piping.
A look inside the resonators—looks pretty free-flow in there.
Compare that to what the inside of the M3’s factory cats look like, and you’ll why there should be a significant increase in airflow, and therefore power.
Here’s a look inside the X-pipe, which appears very smooth.
The test pipes come with two bungs each so that all four factory oxygen sensors have a place to go. This setup is to be used for off-road racing purposes.