"Utilizing the power of DOCSIS® 3.0, the SB6120 enables channel bonding of up to 4 downstream channels and 4 upstream channels, which allows an operator to offer their customers advanced multimedia services with data rates of well over 100 Mbps in each direction."
If you are looking for all 8 downstream channels then you will need to purchase another modem as the 6120 can only bond 4 downstream channels and not 8 channels.
+1 vote for the SB6141. It's currently the best officially supported modem on Cox's list.
Worth pointing out that even bonding 'only' 4 channels will still allow you to easily download at 152mbps theoretical, which is about 75mbps minimum absolute worst case.
What 8 channels will do for you is *if* you are on a congested node is it will allow you to achieve more consistent speeds during peak hours. This only matters if you are seeing speed drops certain times of the day.
For what it's worth, the new CMTS units Cox is installing are each being supported by a pair of load-balanced 10gbps links, where previously it was a pair of not-so-well-balanced 1gbps links. The backhaul should not be a bottleneck for a pretty long time. Channel congestion is still possible, but the most common source of node congestion should be completely gone.