What Ron said.
I could be wrong, but when they say "copper" plug, I don't think that includes the electrode material at the surface of the actual spark gap - I think it's talking about the center conductor going from the top of the plug to the electrode. With an iridium or platinum plug, the iridium or platinum is only a very thin piece of that material bonded to the surface of the electrode used only because of its resistance to erosion from the spark (the electrode with negative polarity wears a lot more than the positive electrode because metal atoms actually get ripped from that electrode with the electrons when the spark occurs - the other electrode has very little wear over time).
The electrode end material has *very* little effect on overall plug resistance because total resistance is the sum of the resistances of all the materials in the current flow path. So even if the electrode surface chip has relatively high bulk material resistivity, it is only a few thousandths of an inch in the total current path that is more than 2 inches in the plug - plus whatever resistance is in the coil wires, etc.
If I'm right, you could have a "copper" plug with one or both electrode spark surfaces either platinum or iridium.
You are correct about copper being a very good conductor (both electrically and thermally) - the only metal that is better is silver. It's funny that people will argue almost violently when you tell them that copper is more conductive than gold. People mistakenly think of gold as the best conductor because it's a premium material to plate connector terminals with (the military uses it) - so they think "Why would they spend extra money on gold if it wasn't the best conductor". The answer is that it's not the best conductor - even aluminum is a better conductor than gold. Gold is used because it is very tarnish resistant, and so maintains good conduction between mating terminals over time due to very low surface corrosion.
Do you have the brand and part number of this "copper" plug? I'd like to read up on it.
Hmm - a quick Bing search:
“Copper spark plugs” is a term mistakenly used for a standard material spark plug. A standard material spark plug traditionally uses a nickel-alloy outer material fused to a copper core. Almost all spark plugs use a copper core center to conduct the electricity, jump the gap, and promote heat dissipation.
"And that's all I got to say about that."