I haven't noticed any performance difference with the iridium. Guess they last longer but I usually change 'em after 20k miles or so. There is 1 NGK iridium which has a fixed/permanent end/terminal cap. It does NOT unscrew. So be sure you get the right plug for your spark plug/coil end. I've owned 950s and a 990 and don't remember on which bike I had the problem or the plug #.
KTMs--300, 518, 990