The Pro-M is probably your culprit. Lean code means that the PCM is trying to add a lot of fuel correction (more than +30% Long Term Fuel Trim + Short Term Fuel Trim). This would leave me to believe that the MAF is giving readings that are lower than they should be.
Take this example:
You are taking in 14.7 "pounds" of air and should add 1 pound of fuel. If the MAF is giving a reading of 7.3 pounds of air, the PCM will think it needs to add .5 pounds of fuel at first, but the O2 sensors will tell it that it needs to add more to be stoich. Once it adds 30%, blamo - CEL.
I bet that Pro-M is calibrated for larger injectors than you are running.
Or, you have some sort of unmetered air leak after the MAF. Same symptoms from a different problem. Or there is a bad connection at the MAF (dirty pin) sending artificially low readings to the PCM.
