AD Sion with ravenous hydra gets massive regen benefits in teamfights and duels in minion waves (like morde). Combined with the Sword of the Divine, he can wreck in teamfights, 3 MASSIVE crits in an aoe are pretty heavy. I dunno though, Sion still suffers from his same age old weakness - get kited forever because he is SO slow with an unreliable gap closer.
Even with the boots that give flat speed, people can get away from him easily.
So with this massive buff to AD Sion, not sure if he will be viable.
Sion is one of my fav. chars, I would love for him to be viable in more comps but... he isn't.