It would be interesting to see where the long (& maybe short) term fuel trims sit at for idle, mid & high rpms. And are the fuel injectors stock size? My tune place (LT1swap) wanted the serial #s for the programming.
I also would look the MAF & O2 reading.
http://easyautodiagnostics.com/gm/4....e-maf-sensor-1
I have a simple phone app & ob2 module (~$20) based reader that can look at real time sensor info. I had a rich condition from my passenger O2 sensor signal wire having a bad connection. Hours of swapping sensors, tracing wires...
What do the spark plugs look like - super white? consistent with all of them? Could point to if you actually have a lean condition or it thinks you have one (less likely).
Does unhooking O2 sensors force open loop? If so you could compare open & closed loop fuel trims. If both sides do the same thing, it would point to something common like MAF or programming, and maybe eliminate O2 or injectors.
Make sure you don't' have a sneaky vacuum leak. You can use a smoke machine or my preferred super unsafe carb cleaner soak down method. Spray, & if rpms increase you have a leak. Keep fire extinguisher handy.
Mildly unrelated - when I first started my 5.3 I had the MAF backwards - she did not like that one bit... When diagnosing these things I don't look for that is wrong, but what is working correct and eliminating it as a fault. Makes me look at the system as a whole, not separate parts.