On the advice of Klaus Savier at LightSpeed Engineering I just installed a set of Denso IK27 Iridium Spark Plugs. Will let you know if I see any difference. At least they are lighter and much smaller.
I have made one flight and can confirm that they work. Need more time to see if they make a difference with engine temperatures.
As you can see in the images below they are very different from the stock Denso plugs.