I have no idea. I'm not in the market for a new modem so I'm not sure. However, I do know there's no upper bound on DOCSIS 3.0. DOC3 handles the data in discrete channels and must support >=4 channels, each with a thoroughput above 40Mbps IIRC. There's no max number of channels and thus there's no upward bound on speed. Different hardware will support different numbers of channels, I am sure.
You might want to read up on information theory: http://en.wikipedia.org/wiki/Shan...ey_theorem
There are only so many channels, i.e. no such thing as an infinite-bandwidth channel.