I used iridium plugs once and immediatly changed them. The bike was less responsive so I reinstalled the stock plugs right away. If anything, install the non-resistor type for more spark or just upgrade to a better set of coils.
From my research on the iridium plugs I've found they are built for higher temperatures to withstand deteriation over time, the material conducts electricity more efficiently which means it take less energy to fire them which is why they are just 4mm in size. The race type installed in cars are 6mm.