The thing is, we actually have no clue as to how powerful Sion is, outside of his 'invincibilty' technique, however the logical assumption to make would be that he is indeed extremely powerful to even be able to pull off such a feat. It's testament to how powerful he really is, and it should be assumed that he's not just some weakling. Now I know that the Exile did completely outclass him in their duel, however it's likely that The Exile herself was damn powerful too. I personally don't see anybody beating him in combat.
Can we even be certain that it requires an amazing level of power, or just enough power combined with an amazing depth of hatred and will to survive? It isn't logical to simply assume that Sion is uber-powerful (on the level of Luke or Sidious) simply by this feat alone, nor The Exile. Sion isn't going to overpower his opponent through sheer strength. In the case of someone like Luke, his only chance of victory would be when his opponent is tired - he can't match someone of Luke's calibre on an even playing field.
If the "breaking will" still applies, then I believe Sidious is easily capable of defeating Sion. His intelligence is, at least, on par with Traya's own, and he is more of a "personal" manipulator than anyone we've seen before. ACstyles has proven that Sion can be defeated through other means than simply mentioning Kreia, so if The Exile - who isn't proven to be a manipulator on any level - can defeat Sion by breaking his will, it is extremely logical to assume that Sidious can do it as well.
And I don't know about that, Malak's pretty powerful, given he gave Revan a pretty good run for his money in their final duel in KotOR, and there's also the fact that he had the power of an entire force sensitive race at his disposal via the Star forge, so he's probably got a force connection similar to someone like UnuThul, who even put up a pretty good fight against DN Luke.
Where is the proof that Malak gave Revan a "pretty good run for his money"? Where is the proof that, outside of the Star Forge, Malak has the "entire race of Force sensitives" at his disposal? Lastly, wasn't UnuThul only a "match" for Luke when empowered by the Killiks?