which brand is your extractor?
if its hotbits, i know for sure they dont want you to wrap it.. it will become brittle and you'll get a gaping hole when water creeps in between the wrap.. saw it happen to another forumer in another forum.. cant recall, it was last year.. anyways.. i use to use hotbits full sytem myself and speedworks told me dont wrap it..
anyways, you should check back with the maker of the extractor.. see if they think your header could withstand it being wrapped..
something worth reading:
Century Performance Center, Inc. :: Exhaust Header Heat Wraps - DO NOT USE