Mind Sludge is amazing in this format considering the limited answers and Sign in Blood is a cheap card advantage card with a small price, alternatively it can be used to kill an opponent in a pinch. You may have noticed I have Hexmage is sideboard for Planeswalkers and RDW if I feel the need for a first striker. I chose Grim Discovery over Disentomb because it allows me to grab a used fetch land in order to reset the top card of my deck for the Nocturnus effect. I am unfamiliar with what you mean by "get a sack in there for walkers." If you mean creature sacing then I have Gatekeeper for that.
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I opted for Urge to Feed over smother because it can almost always handle any 3CC or less creatures and is not such a bum draw late game.