As you say, you have to be a bit sneakier across the pond.
My suggestion, on the 318 anyway, would be to block your upper grill from behind with black ABS plastic, coroplast( or the Scottish equivalent). I started with a rear mounted block and no one noticed that I didn't tell, and did get quite a difference( sorry, I don't have exact numbers as I was making changes quickly both to my driving and car. Also you might want to put on a front belly pan to close up that great hole under your radiator
. Make it easily removable for the MOT
. The Estate would probably benefit even more from these mods.