A while back I upgraded my modem from a DOCSIS 2 to a DOCSIS 3. I purchase my own cable modem to save some money over leasing it from Comcast. It pays for itself within a year. I was using a DCOSIS 2.0 modem and pretty content with the results but then a brand new DOCSIS 3.0 modem went on sale for less than $100 so I upgraded. The difference is noticeable and as you can see from the results below the speed is definitely improved. I have the Xfinity Blast package which is something like 50Mbs down and 10Mbs up.
Also, another great way of testing is torrents since they can chew up bandwidth very quickly. As you can see my download speeds on the CentOS torrent are great.