It's a liquid cooled engine. I don't see how the head can get any hotter than the rest of the engine unless the water passages have a problem.
Also keep in mind that there are plenty of counterfeit spark plugs going around now. NGK's site has a page that shows how to tell if "NGK" plugs are really NGK plugs.
http://www.ngk-sparkplugs.jp/english...ake/index.html