I have the exact same card, sapphire 7950, from tiger direct. I can't get it to hash above 530 KH/s. What's your cgminer config like? Thanks in advance!
What settings are you using?
Most 280x should get decent settings with a few parameters. Change any one, and your hashrate drops.
TC 8192
I:20
GPU Threads: 2
Engine: 1025
Memory: 1500