I'm going out on a limb, but I'm predicting that the sleeper of the set is Demonic Project. I have been messing around with a Warlock Control deck and it can absolutely destroy combo decks, like Topsy Turvy Priest and Maly Druid. Cifka is currently streaming a similar deck and it looks really solid, with good matchups against both Zoo and various combo decks, thanks to Demonic Project.
Sleeper of the set doing what was expected of it before the set launched? No. Warlock was designed to be the combo breaker class. It should lose handily to almost any other control deck though from the looks of it. Watching the same stream as you. For the record. The real sleepers of this expansion are oddly the mechs themselves. They were trashed throughout reveal season and yet you would be hard pressed to go three matches without running into a deck that is full of them. Mechs are good.