I wouldn't use Sims as a marker for DPS on a fight like Mar'gok. He's a very transition heavy fight with periods where you don't deal damage to him and have to switch to adds. The periods of AoE are also pretty weak for affliction since specs like Destro are able to take advantage of shadow burns with so many adds dying. Simcraft uses a long duration stand still fight with little to no movement, and plays like a robot so it doesn't react to situations where multidotting or cleaving could be beneficial since that can gain you a lot more soulshards for haunt.
At your item level, it might be better to lean towards destruction for fights like this and start taking advantage of shadow burns. You'll get a lot more embers, chaos bolts, and overall the mechanics lend better to the fight compared to affliction.