IPv6 doesn't matter. The only way to get a IPv6 connection is to tunnel through HE. If you don't know what that means...then you don't even need to worry about IPv6. What ever DHCP gives you is fine and doesn't effect your speed.
As for your speed, I would say the first step is confirming that the Premier service is active on your account, and specifically the one for DOCSIS 3.0 modems, but since it seems like you have had one the whole time (before and after upgrade) I don't think that would be a issue. This can be confirmed by either calling Cox and talking with their Sales team, or providing your account info to one of the Moderators here. They should also be able to confirm the correct file(the one that dictates your speed) is on the modem.
Once that is confirmed, I suggest resetting the modem and trying a speedtest direct to it, bypassing any network you have. If still having issues, I can give additional suggestions, but lets start there.