i have never done it but a few of my mates have and 2 out of 3 of them got cracked heads on the exhaust side due to the added heat retained from the wrap. mind you they had cast iron manifold and it not recommended to wrap cast iron maifolds as they are well know to crack when heat is retained, i personaly say if u got an after market manifold e.g stainless stell then wrap it but if u got a stock cast iron manifold wrapign it may cause cracking so i wouldnt bother. Anyway the way wraping helps performance is the heat wrap holds the heat in the manifold makign the exhuast gasses hotter which aids better gas flow and reduce the radiated heat in the engine bay