It should be noted that the Warrior never ascends to the rank of Darth in his story. The title is merely given due to outcry during beta that Inquisitors became Darths and Warriors did not. I believe the same was true of Consulars and Knights, with Consulars being granted the title of Master, and Knights not. I could be wrong about that, though, as I've yet to finish my knight's story.
If you check out the codex entry on "Sith Titles" available on Korriban, it says:
"Darth: The strongest Sith Lords ascend to the position of Darth. Many take a new name at this point, symbollically embracing their transformation into something greater."
So, not all Darths choose to take a new name. Most likely Vowrawn did not. Baras and Angral didn't either, I believe, as I recall both being called Lords in Threat of Peace, nor did Malgus (he took a new name before he became a Darth). We know Darth Thanaton did, and it is very likely that Darths Marr, Ravage, Mortis, Decimus and Rictus did, as well, considering their names.
I think it is fair to assume that he surpasses Darth title by light years and he can easily choose to use it - but why would he, hes a Wrath, chosen of the Emperor and probably combined forces of all other Siths wouldnt stand a chance against him.