Thanks Charlie, I think that pretty conclusively answers the question. The speedbumps I am going over daily are at least that tall, but they are the really steep kind, not the gentle sloping kind. I definitely can't lower that much and still clear at the front lip.
Maybe I should stick a little closer to stock. Like maybe some upgraded Koni shocks (http://www.ecstuning.com/BMW-E46-M3-...cks/ES1832099/) with stock springs.