Michael,
IPv6 BGP capabilities has been added from EXOS 15.1, so you need to upgrade at least in this release.
Summit x450a is fully supported up to 15.3, included. The only "challenge" here, especially in the case of BGP, is the memory usage. Summit x450a has less memory than more recent hardware platforms, and as new major version of EXOS are released, the memory footprint is bigger. As BGP often means large RIB, this might be a problem in certain cases.
Just validate your memory usage when you upgrade, to see if that can be a problem for your case. You may want to monitor it as well, for the long run.