I wrote a HUGE reply . . . only to have it all get lost. It was a work of ART. Geeze. Okay, here goes nothing.
Firstly, Dos Santos is fabulous. I've admired his work for a while now. Great choice, and congrats. I'm eager to see what he cooks up.
Secondly, I've got quite a bit to say about the title. It really prompted a lot of thought for me; not only when I first encountered it, but also as I read the story, and as I pondered it afterward. I'll try to be brief, however (Mostly because I just might kill myself if my reply gets lost AGAIN.)
My overall opinion is much along the lines of Ookla and farak. It'll look great on spine and cover, and it fits the tone and feel of the story perfectly. Farak said it so well that I just have to quote it, "part mythological, part epic, and all self confident." Perfect way to put it.
The title is brief and bold, and sparks curiousity . . . a curiousity that the story simply doesn't satisfy until near the finale. By blending it with the titles of the Returned, it loses a bit of the cliche normally associated with titles . . . this title matters, but is part of culture and not merely a plot device.
This is one thing which made me wary when first encountering the book; it's a little ambiguous. Well of course a warbreaker "breaks wars" but what exactly does that entail? Is this something unique conceptually to the story? Is it a common thing? Will I follow the story of this individual? Is this story a heroic, regency, or epic? This latter question is primarily what made me wary . . . I didn't want to experience another story about some "long prophecied individual of the land of <insert world name here>, the savior <insert mythic title here>!!"
As mentioned before however, this cliche is beautifully laid to rest by cleverly prolonging the readers encounter with the revelation of the identity of the Warbreaker until they've both: A) already identified him, or B) forgotten that the title is of importance. The pieces are up to the reader to put together, and not repeated endlessly on every other page. (I actually wish that a countless number of unexperienced fantasy readers will purchase Warbreaker because of its resemblance to countless other epic fantasy titles so they will learn to appreciate the brilliance of a well concieved twist of genre)
Another problem beset me while reading Warbreaker relating to the heroic end of the spectrum: doesn't there have to be a war in order for a warbreaker to break it? Where is the war? I realized that it does arrive (and splendidly, with Brandon's usual everything-goes-to-hell-at-the-end style,) but I found the characters all removed from the gritty effects of war itself up until that last moment. Even Vivenna is really only dealing with civil unrest for most of the story. The characters, however, are engaging and the war seems so distant because I found myself drawn to the characters struggles . . . this naturally makes the war a bit more of a surprise as well. So there is a bit of a trade off there.
In general, my opinion about the title is the same as my opinion about most of Brandon's work: it leads readers in with promises of traditional fantasy, slightly modified . . . and then turns the tables on them, leaving them to wonder which promises were kept, which promises were broken, and which promises they'll trust any author (other than Brandon) to make, ever again.
I'm glad you're not considering changing it . . . I feel that any potential misunderstandings or expectations it might cause will only make the reading more enjoyable.