The headers are " as made" Doug's ceramic coated. And as good as the company makes them, they are production pieces so I'd guess the coating is there for appearance more than functionality. My gues would be they are not coated inside. But I really don't know.
I would guess that with outside temps at 100+ and add in the heat generated just from normal operation, then factor in the heat coming off the headers in a small confined environment under the hood with very little evacuation and I'd bet that you could cook a roast in short order under there in stop and go. Maybe it silly to believe I can have my coolant circulate at 190 degrees when the air surrounding the block is 250 or more.
There is some truth that more heat is generated with higher pressure. Heck I've seen water boil with no heat added. But that's a different field altogether.