I removed it and have no regrets. I loved it with OEM exhaust but when I switched to Akra Evo the cold start was obnoxious. It had a low resonance that was not pleasing. Some other exhaust systems may have different results.
'08 E90 M3 - Alpine White - Sold
'13 E92 M3 - Silverstone II - Sold
'04 E46 M3 - Jet Black - BBS E88, KW Clubsports, Brembo BBK, SuperSprint Exhaust