i have been doing a bit running on a 21 RWHP RE Himalayan , and it has made me realize just how little of my Airheads 50ish HP I actually use in everyday running, as I don't seem to go much, if any, faster on my Airheads.
So yes, on the tiddler I notice the difference a Iridium plug makes, but not so much on the Airheads as i am probably only using half their actual output anyway. And even less of their potential output!
As Ritchie has found out, if you have a well tuned Airhead motor which is performing and being used to its full potential then you will notice the difference, and as they last longer and will probably slightly improve fuel consumption they are still worth fitting.