I recently had the same situation on my warlock. Copeland's, 640 Sandman, but only a 670 SoN. Regardless of fight length, the Sandman was quite a bit better than SoN for destro. However for demo on a 240s fight (Butcher), a 670 SoN was almost a 1k dps increase. If you want to sim your own character, make sure you put Copeland's higher than SoN on the APL. The difference is small, but its still an obvious thing to change.
I'd assume that you'd see similar results with a 685 SoN. My biggest problem was just getting used to another on use trinket with another bind.
http://us.battle.net/wow/en/characte...orlok/advanced if you want to compare gear.