The fallowing post is of my own personal experience as a owner of a DCM-202 and in no way is meant to apply anything else.
I can't go into details, but perhaps I can shed some light on the matter. Cox didn't upload any new firmware to the modems. It was the network the changed, and the modem became partially incompatible with the network. This has something to do with the modem not being 100% compatible with the network to begin with, since the upgrade from DOCSIS 1 to 2, which is around the time they implemented powerboost. The modem wasn't even compatible with Comcast's power-boost at first, and thus the firmware update for the DCM-202 on D-link's website. That change, like this one, came with a "possible" increase in performance. But also, like then, some of the modems weren't capable of the change. The frustrating part is, some parts gave the customers effected free modems. Others gave discounts. I think it had to do with how many were effected, how many of those modems were purchased, and how many leased.
As for the SB6120, I highly recommend it. I also suggest you take a look at your signal levels. The 6120 has a very sensitive tuner, and gave give you a lot of problems unless your signal levels are right on the money. Also, since your area is now DOCSIS 3.0 compatible, you very well may see a performance increase on top of the one you have already received.