UPDATE: I improved support for systems with a lot of GPUs ( > 8 ) and slow CPU: less CPU load, EthMan works fine now. I will release new version within a few days, also some other improvements will be included.
You need to make this miner multithreaded. The CPU's have plenty of power but you're using only one core. The miner can't make use of the 2nd, 3rd, 4th.... etc. cores.