The Hai are quite tough AFAIK
To be honest, I think that the Hai have the potential to having a greater advantage than the Pahlava. Hayasdan is located very near to some of the most important cities of the Seleukidai: Antiocheia, Seleukeia, Babylon, Ekbatana, Persepolis. Take these cities from them, what remains? Some minor holdings in Mikras Asias, as well as the Hellene-low settlements of the east. It's more difficult for the Pahlava.