Well, it's interesting. Yesterday racing traffic home, I was in the drops (with a regular bra!) and didn't notice any banging. But then on my evening ride I got it in my head to challenge another rider on the road. I went into my drops, and then just kept getting lower and lower as I went harder and harder (there was a bit of a headwind so I noticed some aero benefit). And my knees hit harder and harder as a result! I may never find a solution to this dilemma.

PS - I passed him. Didn't stay away cuz I was afraid I'd dropped my riding partner and sat up, but it was a bit of a thrill anyway.