Are you saying Aiakos somehow makes the Leviathan better? You are spending 3 points to make a 9 point warjack "better". I'm not sure how or why better. Aiakos spells don't really do anything for a Leviathan, and sure, it's a focus source for the Leviathan. If you are simply using Aiakos as a source of focus for a warjack in his battlegroup, a Seether, Harrower, or Reaper would be a better choice.
There are only a couple Journeyman Warcasters that are worth taking. Aiakos is a 14/14 5 health model with stealth. If he has a Battlegroup, especially a 9 point warjack, that's a HUGE bullseye on on that model.
This all depends heavily on what else is in your army. In general, taking a
JWC with a large battlegroup is never a good idea. Only a few factions can pull it off, and Cryx isn't one of them.
A better option to Aiakos would be the Iron Lich Overseer, who has jack marshal. Can't hand out focus like Aiakos can, but can gain souls that are spent like focus on a warjack the Iron Lich Overseer controls. Best of all, if the Iron Lich Overseer is destroyed, his warjack will go independent, instead of inert.