We need to separate out the issues here. Iridium v. non-iridium is one issue. Heat range is a separate one.
Understand, now, that the stock plugs, from the factory do have iridium tips. Mazda did this right with the plugs. If you get factory replacement stock plugs they do have iridium tips. All replacement plugs on forced induction engines should have iridium tips, IMHO because of their superior ability to handle high heat and their ability to resist fouling.
Heat range is a separate issue. The standard heat range is best for engines that do not have increased boost or very high flowing exhaust systems (aftermarket downpipes and race pipes), or are not aftermarket tuned to a leaner air to fuel ratio.
So, use iridium regardless of tune or mods.
Use one step colder plugs ONLY if you are running higher than stock boost levels (manual boost controller, etc.), are running an aftermarket tune (AP, Standback, etc.), or have a big high flow aftermarket downpipe/race pipe.
The reason engines modded this way need colder plugs is because those mods will increase the possibility of engine knock. The ECU will see the knock and hold back timing advance, which will cause the engine to not get full benefit of the mods and could have some effect on drivability.
Running colder plugs on an engine that does not need them will not produce performance gains, but will increase the risk of fouling the plugs. Colder plugs pull heat away from the combustion chamber. The tip needs to be kept within a certain temperature, hot enough to burn clean without fouling, cool enough to reduce the risk of knock, detonation and possible engine damage.
I hope this is a bit more clear.
