The benefits of NGK Iridium plugs are:

- Longer Life

- Improved ignition efficiency

- Smoother running engine

If you have a modified engine, you might want to go one step cooler.  A cooler plug is better able to dissipate heat, so is better suited to an engine that is generating more power, and therefore more heat.  If you noticed that your plugs were powder white the last time that you changed them, it might be a good idea to go one step colder.  However, if you don't drive aggressively, the stock temperature plugs may be fine for you.