The irridium plugs are in theory better spark, so cleaner burn.
The main reason folks on here go for irridiums is because you can get them as a heat range lower (or higher, however you want to look at it). On the cooper s, especially tuned ones the heat in the engine is high...and thus plugs that run hot can detonate fuel at the wrong time...causing engine damage. The colder range plugs are more duarable and heat up less so reducing chance of detonation.
There is not much gain in swaping range 6 BMW plugs for range 6 irridiums IMO. However if your going for range 7 irridiums then its worthwhile changing from the BMW plugs.