From a pure atk stat perspective, Jack Frost as a sub for a water team gets pushed up to ~1973 atk. The current off-elements we have that can beat that are Sphinx, Meng Huo, Kyuuki, and Icarus. And Kyuuki's only ahead by ~25; at that point Jack's HP advantage is probably more useful.
Jack's summon effect has a god awful success rate though, so I personally wouldn't use Jack as a sub outside of fulfilling Illuyanka's requirement.

Reiki as a sub... ok, yea, she pretty much either requires you using Illuyanka as main or you need her summon effect (while not having or wanting to use the water SR dragoon) for your damage cut plans.