It does keep the heat radiation down in the headers , but eventually the heat has to go somewhere.
In the past, almost all NASCAR and other racing engine builders and crew chiefs used header wraps for the added power gains and thermal control benefits offered by their use. Problems occurred when these same teams had to replace the headers after each race (NASCAR) due to the wrap being about the only thing holding each header together. Most engine builders, crew chiefs, and definitely header manufacturers do not promote the practice of installing these wraps directly on the headers any longer. They now utilize the thermal coatings that are chemically and electrically applied to the headers. Popular header coating services include Airborn, Jet Hot, HPC, and some header manufacturers now applying the coatings in-house.