My experience is with cars but this applies to all IC engines;
Iridium plugs don't give a better spark, like platinum plugs they just last longer, so they have been adopted by manufacturers to cut quoted ('book') maintenance times, especially where access is a problem. This often means that the first factory-fitted set (fitted dry, without grease) are seized-in when the due replacement time comes! So much for time saving then!
All plugs should IMO be removed, cleaned, checked, and refitted with copper-grease annually.
So it makes more sense to stick to the recommended standard plugs and change them annually....may work out cheaper, and your plugs are always fresh!