Just installed a catless DownPipe from N54tuning.com on my N55 based 2011 135i. I have the stock exhaust.
With my Cobb/PTF custom tune and other bolt-ons, the stock exhaust + DP sounded better (throaty).
This is a short sound clip of a 2011 BMW 135i after a cold start. Car has N55 downpipe (n54tuning.com Performance Line) on stock exhaust, and custom Cobb/PTF stage 2 + tune.
Listen how the sound starts out raspy and loud, but tones down down and becomes throatier with a nice turbo whistle when exhaust flap and secondary air pump closes. The tone down sequence occurs at the 38 seconds mark...
Im thinking about getting Vibrant Performance resonators installed to deepen the sound, and make it less raspy at cold start, and Wide Open Throttle...