There is a long term pending issue: https://youtrack.jetbrains.com/issue/IDEA-83528.
Gradle 2.11 promised to solve it (https://youtrack.jetbrains.com/issue/IDEA-83528), but it still does not work with IDEA 15.0.2.
I traced it down and found that IDEA and Gradle still disagree about the exact notation.
Looking into the source I happily found it is about to be fixed:
It's currently on the "master" branch. Any change to get it back ported to "143" to get it soon with 15.x?
Just a fix for the root project would be great!
May be to extend LanguageLevel.parse() to mach its own.name() ...?