Update May 21: The Slime Devil preview is out now. It's Level 16 and has at-will powers that cause 3d8+11 damage (the same as the Mimic, also at Level 16). I just hope they either give us the table they're using (though it probably won't be difficult to figure out) or at the very least update Monster Builder so it gives the new numbers.
Judging from the MM3 preview of the Mimic, and the Dark Sun previews in the latest Ampersand editorial, it looks like R&D at WotC has boosted monster damage, at least for the higher level monsters.
All of the higher level monsters have damages significantly higher than what their role and Page 42 of the DMG would indicate (6d10+ for Nibenay!).
Considering how one of my themes of this blog so far has been that higher level parties are not sufficiently challenged, this is welcome news.
5e Release dates leaked
5 days ago