No plug ever will make a huge difference by themselves but a quality plug with quality wires, distributor cap/rotor, and coil will make a noticeable difference. Mostly in throttle response and small difference in MPG.
Platinum and iridium are poor conductors of electricity. Why would any one make plugs using these materials? They are very dense and that equals longevity (not performance).
When I upgraded my coil, wires and distro cap/rotor, I wanted to get a plug that would compliment the other upgrades. At that time I knew of only three electrode materials used in sparkplugs: Copper, platinum, and iridium. Copper being the most conductive, I was going to go with some basic plugs until I stumbled upon Nology Silver plugs. Their electrodes are made of silver, which is a much better conductor than the other three. Only surpassed by gold, which would be quite expensive. This tune up was the first thing I upgraded on my XJ, and I noticed a difference. I’m not saying the silver plugs made the difference, but the combination of all the components did. The plugs are pricey and wont last as long as the regular old Champions, Autolite, or NGK copper plugs, and certainly not as long as any of the platinum or iridium plugs, but I am OK with that.
Science says the plugs should be more a more efficient carrier of electricity and that is enough for me.