IMO, the code happening after the air intake install doesn't necessarily mean the intakes are at fault. I personally think there's a small exhaust leak between the headers and HFCs that was small enough to not trip the code until the stillen kit was put on. The typical code for the stillen intakes is P0300 random cylinder misfire. Kind of weird that isn't happening here.
I personally experienced this issue and went through all the same troubleshooting. New hfc gaskets solved it:
Z1 3 Bolt Metal Manifold Gasket for HFC or Testpipes