Thanks all for the excellent info and suggestions. I'm going to try:

• Cleaning and lubricating the IAC w/ throttle body cleaner, making sure it moves freely and clicks when gently shaken from side to side. Lubricated with motor oil.
• Clean the throttle plate with throttle body cleaner (although it looked pretty clean already).
• Contact the dealer about this service bulletin. Found through one of the linked threads:
  • BMW issued a Service Measure bulletin for the Throttle response issue. For details ... look up service measure B12 204 04. The DME software needs to be updated.
• Run BG44k in the fuel for at least another tank (been running one tankful already) to try and loosen the carbon deposits on the piston crowns.
• I debated pouring solvent directly into the cylinders such as carb cleaner or ACDelco top end cleaner but this makes me a bit nervous, so I will table this for now.
• I had some BG EPR on hand so I went ahead and used it (supposed to clean out deposits in the oil system), changed the oil.
• I will try the "Italian tune up" and make an attempt to drive it a bit harder.
• I verified through INPA that with with light throttle application, the IAC position changes. I don't have a helper so I couldn't try doing this on an actual drive, and I couldn't give enough throttle in neutral without over-revving the engine to actually get the throttle to open.