HD6990 do not work with current Claymore miner. And the author clearly stated that he have no spare time to get obsolete GPU running again. At the moment XMR-STAK has best support for HD6990 card but to reach good performance you need to use 14.4 driver. Other drivers suffer from memory detection issue and have poor performance. Expect to reach something around 750-800 H/s for cryptonight v7 algo per HD6990 (two Cayman GPU 375-400 H/s each). The key to optimal performance is to play with worksize, not only with intencity. 18 to 22 worksizes usually have best performance but sometimes I have to stuck with default worksize 8 because of stability issues. Please share CN-heavy results if available.