The main question is how many channels are allocated for DOCSIS. Each channel gets you about 38mbps of bandwidth, though more can be had on newer standards with 4096QAM (if the SNR is good enough to support it). So if there's 4 downstream channels then a max of about 152mbps total down (upstream is separate).
How many channels can they add? Not sure with current DOCSIS specs, but the wire limits are either 600mhz for old systems, or 1ghz for most new ones. So you cold probably get in the range of 166 total channels or 6gbps or so. Of course in reality, some of those channels have to go to TV and so on.
Now DOCSIS 3.1 is adding new methods for operation and supposedly will pull 10gbps down. Not sure how much of that is tested and how much of that is pipe dream but it is what the spec claims.