1 millisecond failover is more than you can hope on a network environment.
However, from your config i can see the stateful interface and the failover interface are identical.
This is something that Cisco advises against as during heavy traffic the stateful traffic information exchange might delay the failover messages, especially when using http replication. You should use different interfaces for failover and stateful traffic.
We have configured the outside and inside Interface with official ipv6 adresses, set a default route on outside Interface to our router, we also have definied a rule , which also gets hits, to permit tcp from inside Interface to any6.
In Syslog I also se...