I would give the card a shot in the x4 slot. If you notice no performance difference it might be a solution. I really recommend turning most if not all of your case fans around to intake, I succeeded in running tri-sli in an air cooler 800D with very good temps (34-40*C idle, 62-68*C load) like that.
Also, another thing you can try is hanging a 120mm fan off of the top card to help push some air at them. Also, try to find a hard SLI bridge as it forces the cards apart a little more allowing more airflow as you can see like in my old rig: