As you say you change plugs more frequent then you can get away with Non Volvo ones but these usually give problems after 5000 to 8000 mls of use whereas you won`t get any problems with the Volvo recommended plug for your application.
Copper conducts better and is generally used in higher-performance/modified engines. In dedicated race cars resistor-less copper plugs are used.
Iridium and platinum plugs are chosen for their longevity only and if the pucker Volvo plug is chosen for your model the correct type platinum or iridium will be supplied as different engines use the different plug type.
You shouldn't gap iridiums because of potential damage to the tips. For that reason and their inferior conductivity, they aren't really used in modified engines. They are usually more expensive also.
If you are running more boost you might gap your plugs a little smaller from std. to 0.7mm Copper and Platinum type can be adjusted ok.....cos it`s the outside larger part that is adjustable so no damage ensues.
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