I have a similar problem with my R9 280x and 6850. With the R9 working, the 6850 produces about 80% of expected shares, but it's acceptable.
I still haven't figured out the reason, maybe a driver problem when using cards with different chips
Well try also Display driver uninstaller to remove all ATI drivers (start windows in safe mode) and install latest 14.8 Catalyst.
With 14.6 I got a few problems both with Sgminer than with other programs.