I would start by checking fuel pressure because that's pretty easy and inexpensive to do. I'd also want to see what the MAF readings are and see if how far off of target they are. If you can swap with a known good one, it's probably worth it. Next, it's time to do a smoke test.

Basically, the three likely possible causes are fuel pressure, MAF bad, or vacuum leak. DISA / ICV could leak and cause a vacuum leak, but they're only flowing metered air, so they shouldn't cause lean codes.

__________________
2011 M3
2006 Sierra 2500HD 4WD LBZ/Allison
2004 X5 3.0i 6MT
1995 M3 S50B32
1990 325is
1989 M3 S54B32

Hers:
1989 325iX
1996 911 Turbo