The theory is the plug will fire hotter with a more concentrated spark and hence burn the mix better. Both are technically not lying but still wrong.
An iridium plug won't do much extra unless you upgrade your ignition system - primarily the coils, to take advantage of that plug.
Even if you do, on a closed loop control system its going to be held to 14.7:1 stoycheometric ratio by the control system. So a better plug isn't going to burn more efficiently, because the control system already controls the mix - unless as mentioned there was an issue before. If the current plug isn't burning the mix properly you should see that on your long term fuel trims - which might be worth looking at before you do your plug swap - just to have a benchmark.
You can try them, and I am sure you can find an iridium that will work fine - but you likely won't see an increase in mileage any better than new OEM plugs would provide - if either provide any bump at all.