I muse that based on Avellone's hype of Sion's TK and the simple raw power required to hold his body together that Sion isn't outclassed by Bane in the Force by a notable degree - if at all. That being said, given most of his energy is being used to simply keep him alive, he's never going to use all that power and is going to get raped like he did against Surik. Same goes in terms of sabers and all out.
tl;dr: Sion is one of the most powerful Sith in history but most of the power isn't there for his fights, since he's using it to make up for countless other defeats, so he's getting destroyed.