The item for enchanting Rune of Greater Twilight Devastation says it should do 2.8 million damage. The Rune of Lesser Twilight Devastation says it should do 1.6 million damage. I’m using Greater Twilight Devastation on my havoc demon hunter and it is regular hitting for 1.58 million damage, and critically striking for about 3.6 million (the extra damage is probably from my talent Know Your Enemy that increases crit damage). I have tested several procs both on the Kelpfist dummy and on an elite mob in a delve, and looked at the damage breakdown in details, and the numbers are consistent. I know that it does reduced damage on secondary targets but all these tests are on a single target. It seems like maybe the Greater Version is doing the same damage as the lesser version because that one says 1.6 million which is very close to the actual damage number I’m seeing ingame from the supposed greater version.
Lesser Twilight Devastation is also doing less damage than its tooltip says, in the roughly ~850k-900k range to a primary target instead of the 1.6M it states. So the issue here is that either both tooltips are wrong, or both spells’ damage numbers are wrong.
EDIT: I’m gonna guess it’s a tooltip issue, as sims are currently using the actual numbers we see in-game. If the actual numbers matched what we see in the tooltip then TDev would be dominating all AoE sims.