Do pure copper banana plugs make a difference in the quality of the speaker sound.

I will say that what does NOT work is hard metals, like rhodium, or solid brass.

You want your plugs to be soft and non-tarnishing, preferably with a spring to them, like banana plugs have or the spring loaded WBT spades.

Anything hard like rhodium or gold over pot metal (like the Monster screw on spades) is never going to have a tight fit, especially when used in nickel plated speaker terminals.