Today, Ubisoft has issued a patch for Watch_Dog that fixes bugs and performance issues. Mainly, it is designed to reduce stuttering with higher levels of texture quality, especially "High Textures". "Ultra Textures" could still have problems for "some players", but Ubisoft suggests that future updates to reduce stutter are in progress.
Without knowing much about the internal workings of the patch, I expect that it addresses hiccups when swapping textures. Loading textures into memory can take a significant amount of time, and overhead, but it is necessary if the one you need is not in there. As the size of each individual texture increases, fewer can be stored in the same memory space, leading to more swapping required (especially when it is difficult to tell what a user can see at any given point in time). Ubisoft might have found a more efficient organization (for lack of a better word that I can think of) for textures that allow "High Textures" to stay below their target memory footprint, but not "Ultra Textures", at least not frequently enough to call it fixed.
Of course, I could be entirely wrong.
This patch also addresses bugs with multiple network adapters, crashes, and error messages. According to Ubisoft forums, it is available now. It is not yet on their news blog, though.