Bridge regression tests were executed, with WITNESS and INVARIANTS enabled.
Performance test shows an increase in throughput from 3.7Mpps to 18.6Mpps.
flame graph before the change: https://people.freebsd.org/~kp/if_bridge/unmodified.svg
flame graph after the change: https://people.freebsd.org/~kp/if_bridge/unicast.svg
This is also running happily on my home gateway.