Is there a table of specs that shows how much amperage is going through that cable when you have the machine working at max capacity?
It would be nice to have actual numbers, but even without them I doubt you need to upsize the cable for an extra 5'. If you wanted to add 50' to it that might be another matter, but as long as the amperage drawn is not being changed I don't think you need to worry about it. Cable sizes generally correspond to amperage and don't change much until you get to substantial lengths. Unless you are already right at the edge I can't see how going from 10' to 15' is going to make a noticeable difference.
On a related tangent, have you looked at the MagSwitch ground connectors? Where were those things 20 years ago? Or why didn't anyone tell me about them earlier?