you can look up the specs for your cable modem online and see what its maximum speed is, and see whether or not you need a cable modem upgrade.
what I have seen so far (April 2014) in DOCSIS 3.0 modems is 4,4 and 8,4 modems.
why am I not getting full bandwidth of the modem?
usually an ISP in the USA throttles the speed on all internet connections and sells those different speeds at different cost rates.
it's called traffic shaping. the practice is not looked well upon. it can be used to implement usage caps I think and prevent bandwidth hogs from being an problem.