Well actually I've recently noticed that your Sith Warrior is addressed as "Lord" in those chapter cards/intros of whatever you call them. So I think its possible he/she may be a Lord, but can easily be RPed as a Darth.
Considering you get the "Darth" title for your efforts, I think the Warrior would be a Darth, if he wasn't the Emperor's Wrath. As Baras' former apprentice and his killer/usurper, I'd say he could have been on the Council if he wasn't already something greater
And later he's addressed only as "Wrath", maybe once or twice as "My lord Wrath".