I am offering a 20000 einsteinium bounty to the first person who can help me solve this problem. I have the exact same rig setup as my friend minus our motherboards and internet service providers. I am running two rigs and he is only running one. We are mining on the same p2pool as well. In a given day I am receiving about 3000 emc2 running with my two rigs while he is receiving about 20000. Each of his payments are 300+ today while mine are only currently 40-45. Please let me know if you need any more info. I will be glad to provide it.
You need to compare the miner settings with him. Is the rejected shares/ reject ratio pretty high? Then you may have set a too high gpu-clock or mem-clock! Try lowering these settings. Sometimes more isn't better.