I have CSL headers, I fabricated a custom catless S pipe out of a E46 M3 section 1, OEM X-pipe and OEM mufflers.
It's not really loud, no drone but it's rasp festival. I kinda like it. The only thing with a catless car : it smells!! You can't stay in traffic with windows open. I think I will put OEM S pipe back on the car with my BMS X-pipe.
If you want to do a custom S -Pipe with no rasp, you can buy Aero Turbine resonator AR-20 or AR-25. A lot of guys use them on E46 M3.
http://www.custom-performance.net/pr...idCategory=253