My +10 Team ( in which we carry random people ) our setup is usually:
Bear Tank
Resto Shaman Healer / Hpaladin
Hunter(myself)
Warlock (Demo/Destru depending on the dungeon)
Death Knight.
Our Warlock is at 871 and he usually keeps up with me on DPS, which is about 1.5-2m dps on trash, with 350-400k on bosses.
But yes, i've met ALOT Of shit warlocks, but yeah i usually wowprogress people before inviting them, aka always taking the one with the best Mythic+ Progress.
Generally bringing in people who have done 100+ mythic dungeons, wih atleast 24 traits.