Prior to Gatecrash I tried out a Grixis Control deck. Let me just say it floundered horribly. I was amazed how much my Scars-Inn UB Control deck relied upon Black Sun's Zenith. Creature decks are running amok and Mortars + Temblor just didn't cut it for me. I was hoping that GTC would add some good mono-black or dimir sweepers, but alas they must yet again foul my dark existence with White mana. Tapping Plains really does make me feel all warm and fuzzy, and that's just gross.
I think you will need some Mutilates, but in order to make that worth it you will also need more Swamps. More swamps means either a full playset of the black shocklands or hitting the RRR of an overloaded mortars gets impractical. Also Killing Wave sucks in a deck like this. As a control deck, their life total never really matters until you have won the game. So the decision will never be tough for them. I would recommend dropping both the Mortars and the Wave and getting a 3-of both Temblor and Mutilate. You will want to shift the focus more toward Swamps more, but that isn't necessarily a bad thing.
More Swamps so adds a few additional win conditions. Army of the Damned is my favorite black-based control win condition. You play it, untap, kill and blockers and attack for the win. If they have a way around it, flash it back and repeat. Another potent option is Griselbrand. Just make sure you have more than 7 life when you play him, as getting the big demon into play and not being able to draw any cards is very depressing.
Beyond that, I like hard counters more than Syncopate. Essence Scatter, Negate and Dissipate would be my preferences. If there is parity or they are a ramp deck up on mana, Syncopate will just be useless. Also late game it is an aweful top deck, and being a slow control deck you will want the game to always go late. Hard counters are your friend. Izzet Charm is the only soft-counter you should need.
Lastly, Augur of Bolas is a find replacement for the Broker. I don't mind a looter when running flashback spells, but without the option to flash them back I would rather have card selection than blind card draw.
But that's just me. Playtest it and let us know how it goes.