|I have MS Manifolds (cast iron).
I had my manifolds coated to retain the heat inside. I did that years ago (10ish) and have not noticed any cracks.
I don't have empirical evidence if it helped with under hood temps.
Typically, you would want the heat to be retained to help spool the turbos faster.
I believe the coatings I used were by Cerakote. I even had the turbine housings coated.