Shadow Priests are adept of resisting the insanity tho. They look at the abyss and then come back without their mind being taken. That's the most intense form of corruption resistance!
If that's so, that makes 'em extra resistant to Xal'atoh: you can't fill a cup that's already overflowing! Xal'atoh will be like "you must kill them aaaaaaalll....." and the Void Elf's internal nightmare will be there all "hell nah Y'shaarj wait your turn"