When you compare Thriller 2001 Special Edition, Thriller 25, and Thriller 40, the gains have increased.
2001 - 1 song, basically cut down to a snippet, and some interview bits.
2008 - Several remixes, some of which I can still enjoy (as guilty pleasures), 1 with actual new vocals. 1 new Song, not actually a Thriller outtake, we'll just call it a Thriller 25 outtake. And some demoes, B-Sides, and rarities, scattered on various singles mostly in Japanese countries..and Someone in the Dark.
Thriller 40 - All of the above, several demoes, 1 entirely new outtake, so far, and a couple of obnoxious teases. Basically 2 or 3 new entities and then other stuff to pad it out. Still, overall, a lot more than Thriller 25 and basically a true project over the Special Edition.