If you want to try Iridium plugs, there are two (relatively) commonly-available types, both NGK:
LMAR8AI-8 (same heat range as original, 0.8mm air gap). These plugs are original fitment to the K1600GT/GTL/GTL Exclusive.
LMAR9AI-8 Exactly the same plug, just one heat range colder. I use these - amongst other things, they help combat high-rev, high-load pinging. These are original fitment for a KTM 350cc application.