I had my exhaust manifolds, pipes including up and down pipe, and turbo turbine housing coated. The manifolds were coated inside and out, in the past things would get so hot even the hood was hot as hell. Now its cool (relatively speaking) under there even after driving a ton. Turbo spools a lot faster too, I am running pretty lean on fuel, EGTs now only get to low 600s at most now with boost topping 8psi but I can floor it, hop out, and touch the manifolds. All this on an old non-wastegate banks turbo

. I remember when that turbo setup was completely stock, it is worlds better now ( I also changed the intake as the stock unit was poor).
Coating everything up to the turbo increases efficiency. Any heat lost preturbo is power lost. And the less under hood temperature you have the better, thus why I included the down pipe. Plus it wont let the metal rust like it does with header wrap.
The coating is thin enough that you don't need to take any off... I've often thought it would be neat to coat the heads since they are aluminum too, but I don't know if that could problematically affect the thermal expansion with head gasket sealing. Plus there are those that fear if you run too cool you could have flame out. But considering stock compression is over 20, I don't think it would be an issue.