I found something from Hot Rod Magazine. They did dyno testing of an engine before and after applying coatings. It looks like the engine gained about 2% torque at 2100 RPM after adding the thermal barrier.
A Look at High-Tech Engine Coatings and What They are Worth - Hot Rod Magazine
It would make sense that this should be more effective when the engine runs slower because of the greater time to transfer heat. It should also be more effective for engines that have a large bore and short stroke since they have more surface area to absorb heat.
The engine would require less air for the radiator so aerodynamics could be improved too.
I actually did something like this on a Plymouth Horizon. I painted the pistons with white VHT paint. I saw that tip recommended in a book about Chevrolet racing engines. I'm not sure how that affected my fuel economy. I got 44 MPG on the highway with the 1.7 liter engine and the 4 speed "Horizon Miser" transmission.