Blizzard has announced that Xal'atath's Guile is now working as intended and level 12+ keys are doing 20% more damage.
The level 12 affix Xal'atath's Guile has not been working correct since it was implemented, not providing the 20% bonus damage that it's supposed to. The community recently found this out and Blizzard has now fixed this bug before the launch of War Within.
For further context, this change was focused on making the 2–11 keystone experience better.