The M3 exhaust is laid out in the following pathway:
Headers - Section 1 - Section 2 - Section 3
It's designed by Arvin-Meritor and their white paper can be seen here:
http://www.unofficialbmw.com/images/...ust_System.pdf
Headers:
US cars came with the 200-cell cats built into the header contain 1 EGT bung, 2 pre cat and 2 post cat sensor bungs.
Euro cars came with headers without cats built into them. The factory cats are located in section 1.
Section 1:
This is the section immediately after the headers, and consists of a pair of parallel S-pipes.
US cars came with only one resonator.
Euro cars came with 200-cell catalytic converters and EGT bungs built into section 1. CSL euro cats are lighter due to thinner piping for weight saving purposes.
Section 2:
Longer portion of the mid section that connects to the muffler. There is a factory H-Pipe crossover in the beginning of this section, and a tiny straight through resonator halfway down on this section.
Section 3:
This is the actual muffler, including tips. The stock muffler weighs 56lbs and the CSL is claimed to be lighter 5-10lbs lighter. The muffler is not valved internally and is a very good flowing muffler. It features perforated tubes and one crossover chamber.
http://www.m3forum.net/m3forum/showthread.php?t=484858