Mid-game Exile had no chance of winning against Sion on Korriban.
https://www.youtube.com/watch?v=sDjhxG4GFfo
He cannot be defeated... he is not a beast of flesh and blood. This is not a battle that can be won. Flee.
―Source: Knights of the Old Republic II: The Sith Lords
You don't have to fight for long, however, before receiving a telepathic message from Kreia. She warns that at your current strength, Darth Sion is unbeatable; your only hope is to flee.
―Source: Knights of the Old Republic II: The Sith Lords: Prima Official Game Guide
Not very. She grows in power immensely towards the latter stages of the game by actually fighting and killing lots of Force-wielding Sith. Beforehand, the only Force users she kills are two Sith Lords on Onderon and a bunch of shitty Assassins.
Chances are that Kas'im beats Sion blade-to-blade, but Sion would keep rising up and strike him down eventually.
Originally posted by crackshack2
How powerful was midgame Exile?
It's hard to say exactly how powerful the Exile is at this point, since there's few concrete showings and most of those take place before on other worlds, where we know the Exile is constantly growing stronger. We do know that much earlier in the game, she's beaten Sith Assassins who were on Peragus.
Early-game Jedi Exile outdueled all five handmaidens at once.
Others of her Telos feats are surviving a shuttle crash and then proceeding to cut through a small army of Czerka mercs and beat a tank droid and then tanking another shuttle crash and shortly afterwards beating 3 HK-51 droids.
Visas was the fourth most powerful and skilled Sith in the entire Triumvirate, an elite sect of assassins who were highly trained war veterans as Revanchists before gaining the esoteric techniques of the Trayus Academy. Also she incapacitates the entirety of the Exile's party before she fights the Exile. And yet she was "summarily beaten" by an early Exile.
And again, this stuff is all well before Korriban.