I pulled the Iridiums out of my newer stuff and put copper equivalents in after wondering about this stuff my self. My results seem to agree with what others have found on other forums..
They all ran noticeably better, Especially my little turbocharger hooptie.
Copper is a better conductor of electricity AFAIK.
My conclusion is the Platinum, Double Platinum, Iridium plugs provided progressively longer replacement intervals. Period.
Street car, beater, driver, which is a PIA to change plugs, the longer life stuff is probably the way to go if you don't want to change plugs..
Performance, race, use the copper or a race plug. And figure on replacing it once in a while.
And some of the performance was most likely due to the fact that it takes less KV to fire the old style plugs. If you ever find a good one the old spark plug cleaners that had a window to check the plugs under a load with air pressure really tell the tale on spark plugs.
