From his descriptions last summer, he had not done so much work on ToM either before handing TGS over to Tor. Some, but not a massive deal. A few chapters's worth.

If you remember, TOM buids up to a "last act" (typically with RJ that could be a fourth or a fifth of the book) reuniting the main cast as TG itself starts. Brandon was not writing "the reunion" itself as part of his clusters, the stop mark for all clusters was before "it" (a lot of people expects the "reunion" that starts TG will be Jordan's version of the Arthurian "Battle of Camlann" in and around Caemlyn - a theory strengthened by an off-hand early comment by Brandon, months before he spoke of the "reunion", that Elayne had less chapters in the first two thirds because unlike the others was where and how she needed to be for the beginning of TG, that the "wrap up" before TG Jordan had pretty much dealt with in KOD in her case).

Brandon said he had some left over material after TGS for two story lines (we know now it's for Rand and Egwene. He didn't want to say at the time) that he expected to turn into 3-4 "bridge chapters" each between the end of TGS and the big event (about as much as the other two - Perrin and Mat - would get in TGS is how he explained it). He had barely started on Mat's or Perrin's story line when the decision was made to split the book in three and had not touched the other or the fifth cluster (Elayne, BT & misc.), and the story to be told in ToM is the section of AMOL for which Jordan has left the least material of all. Egwene was the character for which he had left the most stuff, then Rand.

As you pointed out, Jordan's near finished material was mostly the prologue (split in two now, with some reworks) and the reunion and TG. Brandon hit bump roads in TGS and ToM that slowed him down (false starts with the Mat and Aviendha chapters in TGS notably - Harriet had him rewrite Aviendha a few times as she found her off character) and then again Brandon struggled to get Tuon's "voice" right too. He also had to spend much time restructuring his two first clusters as a book, and to write the extra chapters he had not got to yet (either Mat's or Perrin's, he never said). For ToM, it's a story line that apparently required important reworking (again, these are the story lines for which RJ has left mostly just notes and scene outlines). Despite that, he delivered TGS on target and apparently will deliver ToM on target too. By this summer, with his prior experience with all the main and most secondary characters and two full books behind his belt, I'd say things are looking fairly good for AMOL, though I wouldn't be suprised they take a tad longer to edit that one since it's their last chance to make sure everything that needs to be there is there. Fans tried to make him comment on the issue of the last book and if it might come out earlier than planned last year, but at the signing I attended Brandon just shrugged and answered those questions with something in the vein of "you never know, Mr. Jordan has left a lot more near final material for (the third book) - including all the climax section and even the "famous" last scene, but I have not really analyzed that part much for now so I can't really evaluate out of the blue how much work I really have to do."

Another comment Brandon made at some point was that the real heavy duty work would be to bring all the story lines together for TG - a section that was full of holes and decisions to make between the As and Bs, that the last third of the book was easier, after the "reunion" he will have to build up to the point from where Jordan himself takes over. We also know that a result of the way they decided to streamline TGS around Rand and Egwene's core story lines (leaving related plot threads hanging for now like Mesaana & Alviarin, the BT, Logain, the Rebel envoys to Taim and so on) is that ToM became a much more complex book than TGS to structure, dealing with many more big and small threads and characters and going in many more directions than TGS did. It sounds like the book that had (has? we'll know soon) the greatest chances to go beyond its deadline is ToM, not AMOL. Unless there are more bump roads, of course.