I think the OP means he's rubbing on the rear fenders, not bottoming out the suspension. Lowering springs will only exacerbate the rubbing issue. Best thing you could do is get a fender roll with a mild pull as well.
FYI for what it's worth, the bmw suspension is designed to hit the bump stops, and work with them to give you a firmer ride and better handling. A lot of people don't know/realize that its designed that way from the factory.
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