My understanding is the Iridiums are recommended as they're meant to last a nice long time (somewhere around 100,000 kms). The fact is, as we're not running the recommended fuel for our machines (cos' it's not available), they run rich and foul the plugs much quicker than they should. Considering you're gonna have to clean them (which you shouldn't do with Iridiums - you remove the coating) or replace them, it's much cheaper to get coppers at ~~$25 then Iridiums at ~~$125. I change mine about every 20k... dunno what the recommended interval is, but by the time I pull them out to look at them I've replaced them anyway.
Oh, and copper conducts better then Iridium too.
Now, the recommended plug is a 1.1mm. If you need to go to a 0.8mm, it's cos the spark isn't strong enough (tired coils, more boost, etc. not allowing a proper spark across 1.1mm). That said, going to a 0.8mm is an easy way to fix problems - I do this on my 33. Probably the quickest fix if your machine is coughing & spluttering at high revs under boost.