Are you running powered risers? I have 5 Dual-x's and I'm getting around ~730 kh/s. You're gonna have to tweak the settings around to what the cards like. I'd try putting the voltages around 1.1, core clock @ 1040, and I found out my Dual-x's like thread concurrency 11200. After you get a stable baseline for the cards, that's when you can start tweaking for better performance. 10 more hashrate is going to produce more if you're getting rejects and errors