Yes I did this, it makes start up quite a lot louder (the low notes, almost making it sound 'blown'). No perceptible change in motorway 'drone'. It also makes the loud crack you get on WOT at first start up much louder and start sooner.
I unblocked the pipe eventually as the noise at start up was too loud and sounded a little rough. However I've since had to block the pipe again as the flap in the exhaust is now rattling at startup/load with it set at factory (unblocked) and under insured warranty, its not covered by BMW, therefore the cost to replace will be including the entire back box on that side. I'm not doing that, I'd rather just disable it.