Try a lower thread concurrency. I have a 7870, and my best results were with about half of that.

Drop your concurrency to 1/4th of what it is at, and intensity to 10.  Then, move intensity up, 1 at a time, to 13 or so.  After that, go to 1/2 what your concurrency is now and intensity of 10, and again move up 1 at a time to 13.  Record your hash rate at each point.

That will give you the best concurrency \ intensity combination for your card.