Too lean sounds like a vacuum leak. Un-metered air getting in past the MAF's. Vacuum line, Intake manifold leak? MAF's for leaks (I know I mentioned this before and you said you checked them but....), O2 sensor could cause one but shouldn't cause both banks to show lean. Vacuum leak can cause both sides as shared plenum. MAF's don't go bad at the same time unless it is a connection/seating problem, O2 should only cause the reading on one bank as it tries to correct the mixture for that banks injectors to match up with the air read from the MAF's.
Nubcake might be one to know for sure in the tuning, but since it is a shared Plenum, a difference from one MAF to the other for pressure difference from one Turbo or Intercooler leak on one side shouldn't show up in one bank or the other, it should show up in both banks since the Plenum is shared. O2 is reading one side so it affects the fuel injected on one side.