To reply to myself:
Turns out it doesn't matter, at least on the 3. The mirrors don't adjust out far enough. Almost daily I'm surprised to find a vehicle in my blind spot when I do a shoulder check to lane change. The large pillars don't particularly help (I had never owned a 4 door car until 2 years ago; thankfully this isn't my first or it would be even worse!)
Still need to find an S to compare with, very tempted to pull the trigger on euro S glass to give it a try.
Since I haven't lived with an S on a day to day basis, I can't say whether or not I'd have the same problems with the S mirrors not going out far enough.