The OP doesn't say in which country he's a resident, or what he plans to do with the coins once brought into the US. Let's assume for the sake of discussion that he lives in the US, and acquires the coins abroad and brings them back to the US.

I agree that as far as the currency declaration, he doesn't have to declare them: he's bringing in CAD500 of legal tender, which is well under the limit. But, then wouldn't the value above the legal tender value be "goods" acquired abroad, subject to declaration and possible payment of duty if above $800?

It would seem to me that the coins have to be declared in one way or the other: either they're currency at the full value over $10,000, or they're goods acquired abroad over $800.