I have not had my 2008 XTR for long when I replaced the plug with an iridium cr7 plug which I got of Ebay I fitted it I found that the engine did not preform as well as it did prior to changing it. The engine started ok etc but before the bike would happy to run at 45mph in top and accelerate without hesitation but with the new plug the engine had developed a flat spot at the same speed on the same road.
When I got back home I check the original plug that came out of the bike that's when I found that it was a CR8 that was fitted. I swapped the plugs and she running sweet again then tried the new plug again and the flat spot was back.
Since then I have done a bit more checking by going onto the NGK web site I now believe the plug I got of ebay was a counterfeit. How you can tell the difference the original has Russian description on the box the plug comes in and counterfeit has not other than that they look the same. you can get all the details off the NGK web site, it is worth a look So heaven only knows what grade the counterfeit was.