I installed and removed a 2018 manifold from my '15 for the same concern you've listed here. I checked the manifold for leaks, and plan to reinstall with tune adjustments. How did the last tune you tried (cmcv delete final.hpt) work out for you? Still hesitation/misfire below 2500?
Rather than start a new thread, I'm going to attempt to revive this one. If folks prefer this die, please let me know, or kill it. Kill it ded.
I vacuum checked my manifold (which was exhibiting the same symptoms K44 detailed) at find no leak. Before going back and reviewing this thread I attempted to set all Mapped points in ECM 38155 to see IMRC as open all the time....the car would startup, stumble and die. I will make the recommended adjustment (thank you ishyne22) and report back. I don't know if anyone cares at this point, but hopefully this bears fruit for someone in the future, and can give credit to those who are due.
You guys run locked imrcs way different than I do. Since the imrcs are locked open. I run mp 14-25 and completely disable 0-13. Cut the number of mapped points in half and the work in half. Real good when tuning boosted vehicles that need changes to torque/inverse.