new modem supported by cox - 2.5G (maybe Gigablast times 2.5?)

A Gigablastier plan?  Probably no time soon.

This is a "Multi-Gig" port.

There used to be 2 Gigabit-Ethernet standards (1-Gbps & 10-Gbps) with no standard in-between.  Perhaps the technology for 10-Gbps got cheaper so manufacturers took a 10GBASE-T port and clocked it down by 3/4 or 1/2 for 2.5 or 5 Gbps.  Or, perhaps, the chips for 10GBASE-T are still expensive but the port itself is cheap and the current chips for 1-Gbps can still support a clocked-down version of a 10-Gbps port.  I don't know.

If you subscribe to gigabit service and your provider throttles up your bandwidth based on "excess capacity," your modem wouldn't be limited to only 1-Gbps.  This scenario wouldn't be a probability but instead a possibility so it'd be nice if your DOCSIS could handle the temporary increase.

It's a nice feature but I wouldn't pay extra for it unless my router also had a Multi-Gig port.