CGMINER usually does need much setup. Just add -I 9 to the end of basic pool information, and that is usually enough to get the highest hashrate (assuming you already overclocked your card)
As for myself, my 6870 is at 290MH/s with only a slight factory overclock (bascially no overclocking at all) on CGMINER.
Try Diablo Miner you should squeeze 300 or so out of that card
I am only getting 280 with Diablo with the settings on the first post. Are there any better settings I should try? (When I did overclock test long time ago on phoenix 1.4, I easily passed 320 (maybe 330 but it has been a while), but the energy spike is not worth it imo)