(only theoretical numbers here)
Shadowcraft has me pulling 102k in
my current gear as Assassination (Mut reforges, Combat gems), and 98k in full Combat gear/gems/reforge w/ H-Gara'kal fully upgraded and N-Spiritsever upgraded and gemmed. Take that how you will.
Off the top of my head, I was pulling 78k on Qiang (Heroic) as Mut w/ Combat reforges while pulling 72k on the same portion of Heroic Spiring Kings as Combat.
I'd say at the heroic raid gear ilvls, the two specs pull VERY close to each other on single-target, with a slight edge to Assassination depending on the mechanics of the fight.
As others have said, Combat will be pulling ahead next Tier for sure; Mastery can only scale so high right now for Assassination before "that extra percent of Mastery" becomes kind of meaningless.