To get the same protection value, steel is honestly your lightest and most efficient option. That's why it was used. And asofar as quibbling with the dates of the Visby find (1361), that style of COP was viable for use until around the early 14th century, considering the "armour lag" that styles took getting all the way to the Baltic.
If you are going to start throwing additional protection in top of your maille when your persona is in "The Age of Maille", I'm assuming you're doing it for protective value in combat. Cause let's face it, maille kinda sucks at absorbing all the blunt trauma we face (being literally the only threat we face, unless something has gone horribly, horribly wrong) and I will use every piece of rigid armour I can justify. From a practical (not getting hurt) standpoint, the Visby style is awesome. I've worn it in combat, and it has awesome movement, and it protects very well.