Thermal coatings will keep the heat in. Swain's white lighting thermal barrier has gotten good feedback.
Ceramic coatings are good protection, but don't stop heat like a true thermal coating.
Generally, coatings work with radiated heat, they wont stop conducted heat. ie From being attached to the block like an intake manifold.
Turbos and headers are the favorite things to thermal coat. I don't know to what exent thermal coating helps their performance, but it certainly cuts down on under hood temps which helps with longevity of other components.
Naji did a CAI wrap experiment.
http://www.geocities.com/n_dahi/ click on cai wrap.
I don't know of anyone who has coated their cat.
This is all IIRC. If any of this is monkey, I'm sure someone will correct me.
