What are you fan speeds? Ramp the fans up! We run ours at 80%.
I am running mine at 85%.I also have 'auto fan' enabled.
I lowered my hashing speed to 670-675 by decreasing the gpu clock to 980.Now it runs at 78 degrees.
We got a bunch of Gigabyte 280x at the office, and I have one at the house in my gaming rig. With the F70 cards (which means Hynix memory), I can run 806kh/s with 1145/1810/1.200v @ 90% fan for 78C. Either you need better airflow, or you need to undervolt your cards.
Most of the cards at the office are F60 (Elpida), and run 743kh/s @ 1070/1500/1.09v with 80% fan, and are usually in the mid 60C range.