Having a DOCSIS 3 CM has nothing to do with providing IPv6 to the CPE. DOCSIS 3 added IPv6 support for the CM itself. It could care less what flows across it. The CMTS on the otherhand is another matter. IPv6 support can be configured on a cable interfaces of a Cisco uBR which doesn't yet support DOCSIS 3. The uBR7225VXR is a good example. Other vendors are different. Arris C3s do not and never will support DOCSIS 3. DOCSIS 3 isn't a simple software upgrade. It has specific hardware requirements. Arris' C4 on the other hand is modular and can have modules with DOCSIS 3 support installed in it. Motorola's BSR line is similar. The pizza box CMTSs do not support DOCSIS 3. The larger modular chassis like the BSR 64000 will eventually support DOCSIS 3 (they currently don't or didn't the last I looked).
On the topic of DOCSIS 3 in general, there is no such thing as DOCSIS 3.1. There also is no such thing as "proprietary extensions" being added to DOCSIS 3.0. The notion is purely horse shit. A CM or CMTS is only certified by CableLabs as DOCSIS-compliant if it adheres to the strict CableLabs DOCSIS spec. There aren't any "proprietary extensions" in DOCSIS. This is exactly what DOCSIS was created to eliminate. CMs are 100% interchangeable. Moto CMs offer no enhancements on DOCSIS that Arris CMs don't already offer and likewise with the other smaller CM manufacturers.
The only things slowing DOCSIS 3.0 deployment are 1) demand or need for the benefits of DOCSIS 3 (bandwidth or IPv6 support for CMs) and 2) cost. DOCSIS 3.0 requires a significant investment in the HE. It's not a simple software upgrade. In many cases the CMTS itself must be replaced and a new investment made in expensive new hardware. If the cost was the same then we'd be deploying DOCSIS 3.0 CMTSs today and migrating CMs as needed. An Arris C4 costs approximately $250,000 for a basic chassis with half a dozen upstreams and a single downstream. The pizza box C3 which does not support DOCSIS 3.0 (and never will) costs $20,000. If the demand was there for bandwidth we'd be deploying DOCSIS 3.0 CMTSs. If we were Comcast and had the world's largest CATV OSP we'd be deploying DOCSIS 3.0 CMTSs for our CM management needs alone (and they are BTW regardless of whether or not they're offering DOCSIS 3.0 to their users).