A quick question: Does exhaust come out that hole when it is removed. If so, I would think you would be changing the tuning of the way exhaust is meant to go down the pipe. This gives an easy access for the exhaust to escape, but not neccessarily at the proper point. You actually could get less performance out of it. If a LOT of exhaust comes out that hole and there is not enough pressure to force out condensation from the heating/cooling down process, the back half of your exhaust may rust out over time. If exhaust doesnt come out of there, then you probably are ok.
I am sure BMW wouldnt have spent R&D money to make this piece if they didnt feel it was important to have on there. I know in the car audio world, adding weight to a panel makes that panel harder to rattle from the bass speakers, the whole idea behind sound deadening. I would search around a little more.
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