Nowt wrong with Denso the only plug that lasts in mine 5k miles & numerous 200+ runs at 2.5bar. Can i suggest respectfully that whoever specced your plug grade got it wrong. If correct Denso plugs are used they are simply the best. Ive seen the grade that other big bhp Cossies use & they are miles away from the correct grade & will give probs, unfortunately i cant give this info away.
Rod