Good question since people get really excided over class rings which have a low melt value but look impressive. Those come in well above nickels. Wedding bands vary quite a bit too, I have a rose gold one that is up in the flip tab range. My most valuable rings are well below nickels and those have high end diamonds, sapphires and one has emeralds.

Guess it's subjective.
Only thing my test proved is that copper content is a big factor.

What might be a good test not using nickels is see which machine can pick a fringe target like a chain? Chains I think are the toughest to find even on land. That test will prove how close IB machines have come and their ability to handle salt. I wouldn't use a super fine chain as even pi may struggle even without the salt right?