Functionality wise any RouterBoard device with ipv6 package installed (ROS v6) or running v7 is equally fine for IPv6. Capacity wise one should keep in mind, that ROS so far doesn't implement fasttrack for IPv6. So if configuration for IPv4 doesn't enable fasttrack and router is capable of required performance, it should be fine for IPv6 traffic as well. If fasttrack is required to get to required performance level, then that device is under powered for IPv6.

Performance in IPv6 and IPv4 can differ a bit since there will likely be differences in configuration: NAT is not needed in IPv6 in most of use cases, firewall rules are slightly different (probably thEre will be more in IPv6). Overall the performance difference between IPv4 (without fasttrack) and IPv6 should not be huge. Probably in the range of performance degradation of IPv6 due to larger packet header.

When talking about CRS devices: on L2 (switching/bridging) there's no difference between IPv4 and IPv6 (or any other L3 protocol run on top of ethernet).