I tried iridium plugs in my Norton Commando race bike. On the dyno they showed about 1 horsepower improvement throughout the range.
However.. my drag racing guru friend reckons that if you get a gain from something like different plugs, there's a fair chance that your ignition system is the real problem. You might have a coil on the way out or other HT problem.
The trouble with the world is that the stupid are cocksure and the intelligent are full of doubt.