I have the M Performance Exhaust too and think it was a good investment, though honestly speaking, a bit too quiet for my tastes (I sold my Megane RS Trophy 265 with KTec Sports exhaust to buy my M135i).
I agree that it sounds much better with the windows open, allowing you to hear a little bit of crackle and pop on overrun (perhaps I say a little because I am comparing it to my Megane).
Not much change at idle, though this is what would be expected for most well-done sports mufflers (Supersprint being one of the best examples for BMW straight 6 engines). But where you really hear and feel the difference is at anything above 4000rpm where the M Performance system really enhances and sharpens up the straight 6 howl. Though peak torque is available at an astonishingly low 1300rpm which means that you don't really need to rev the engine, the sound is so good with this muffler that I find myself pushing right past the red line to about 7200-7300 rpm just to enjoy the sound!
Its a well judged unit that accomplishes what it set out to do - enhance and highlight the sound of a great engine made too quiet by use of a turbocharger, but in a way that continues to allow it to be driven as an upscale cruiser.
Like Porsche's expensive aftermarket sports exhausts (I had one in my 2010 Boxster S), the focus is far more on improving and enhancing the sound of a great engine for those like me who care a lot about the engine note, than on increasing performance and hp figures, and in that at least I think M Performance accomplished their goal quite well...