I think the only reason gold is used for plating is that it oxidizes less readily than the other metals. I haven't done any listening tests myself, but I think that using any form of plating that is of a different metal than the spade itself would have some effect on the signal. I'm interested to see what others think as well.
Does the quality of the spades matter
If you could chose between thin gold plated or thick silver plated over copper, which would be preferred?