Its funny how some people hate the sound of catless m3's and others as myself think it is absolutely heavenly. If you like a race-car sound than you simply can't beat it.
Go watch some youtube's on the DTM m3 race-car and you will hear that its identical to what your car sounds like.
Maybe if you change your perspective and realize the raw raspy note of the exhaust is the sound of a completely unhindered 8400 rpm s65 engine at work-mechanical noise at its best. High pitched and raspy, not unlike ferrari's etc.
If you hate it than really adding another muffler won't do much except make it louder and alter the tone a bit.
If you hate the sound just reverse back as the 15-20hp is not worth hating the sound in a vert.
I mean if you are not in heaven when at 7-8400 rpms listening to the sound than you made the wrong move. The best part of the experience in this car is redlining it to hear the sound.