According to diablowiki:
There are three types of elite monsters in Diablo III. Superunique [...] All quest monsters, including Diablo, are considered superunique. Rare [...] Champion [...]
If I'm understanding this correctly it means bosses are affected by the "extra damage to elites"//"less damage from elites" affix some items have.
Are the Übers and Key Wardens, despite not being regular quest monsters, also elites and, thus affected by these affixes?