This is how I start my xfx 7950 in a mixed card machine. May be you can give it a try. The low core clock and memclock is for keeping it cool, so feel free to up them if that works or you.
./cgminer -o somepo.ol:3333 -u me -p x --scrypt -w256 --vectors 4 --shaders 1792 -I 13 --gpu-engine 800 --gpu-memclock=800