Way back when my LIKE200i had just a few miles on it I replaced the plug with an Iridium. Did the same on the Burgman and the Piaggio when they were new. The only reason I did not make the change on the Forza was because there was no Iridium version.
In each case I noted a smoother idle going from new to new.
By far it is the least expensive, easiest, mod to three scooter engines (and two Scions) - with noticeable + change in my limited experience.
Plus...threading in these tiny plugs in alloy engines always makes me a little nervous. So, the less often I have to do it ,the better I like it.
If I notice no other performance improvements - $10 once every few years to obtain a nice, more stable idle, while sitting in heavy traffic on my scooter is, to me....$ well spent.
My cheap tachometer did pick up the change in idle rpms, and rpm steadiness - when replacing the new NGK with new Iridium NGK in my new LIKE200i engine. Also, my new Burgman never died at a light again after making the change. True, not an unknown, and a temporary issue for new Burgman engines - but I only let it happen once before parking the Burg till I bought the Iridium plug.
Stig