I've seen people put holes in the aluminum housing, but i think that's more for the look than anything else. I'd imagine that one could pare some weight from the ring gear, like you would with a flywheel. That said, when I pick up a BMW final drive, I feel like it is pretty light, for what it is. Go pick up one from a Guzzi, it's much heavier, and then you'll be happier with the weight of the BMW part!
I wouldn't bring her home to Mama, but Mama ain't home tonight.