The percentage of heat lost is all dependant on the material of the manifold, backpressure, and distance the exhaust flows.

Heatwrap can prevent a good amount of heat from escaping but if you have a short, log manifold, it won't do much. It all depends of the shape and material of the manifold on the gains seen.

My turbo manifold is cast iron and very heavy and thick. It is also a log manifold, so I probably wouldn't see much gain out of heatwrap.