Is Korriban running through Sion? If so, it's hard to see him losing. He can simply keep reviving himself like he did against the Exile.
Meetra Surik is within Obi-Wan's tier of skill, and thus should be close enough to Windu. In fact, given her superiority to Kenobi as a Force user, she should be even closer to Mace than he is. With that in mind, Sion was capable of Force Crushing Surik and giving her a well-contested fight, so he shouldn't be far off Mace's tier of skill or power. Add to that constant ressurection, and Windu's inability to answer Drain, and the victory should go to the Lord of Pain.
I'd imagine it's a good fight, though.