Exhaust wrapping has nothing to do with cooling drag, it simply keeps exhaust gases hotter which may increase engine efficiency, its rather complex but I suspect the wrap in this instance is simply to stop burning the closely fitting cowl.
Cooling drag is all about the reducing the drag from the airflow that is used to cool the engine, in other words careful design of inlets, outlets and the flow between these points. theoretically cooling drag can be reduced to a negative figure I.E. thrust.