I also have a Barts GPU (6850) and I went through the exact same problems as you.
How do you make your memory clock stay at 300mhz? When I set the memory clock to 500 or any other clock in Afterburner and then turn on the miner, the memory clock goes up to 1000 again. I've also tried changing the memory clock while the miner is running, it doesn't seem to make a difference. I did manage to raise my core clock and got a few extra mh/s.
Also, I set my workload to 128 (-w 128). Try it and tell me if it makes any difference.