That's weird that you only get 280mhash/s it that thru pool mining or solo?
I've a non-reference Sapphire 6950 1GB unlocked and I get about ~340Mhash/s @
800/1250 with command lines -v -f30 -w128 thru BTCMine
when I overclock it to 900/1250 (@stock voltages) with command lines -v -f30 -w128 I get about ~385Mhash/s thru BTCMine
That's with running 11.5 Cat and SDK 2.4 in Windows 7 ultimate using guiminer/poclbm 05-21-11
unlocking non-reference cards is simple....
just follow this link to Techpowerup http://www.techpowerup.com/articles/overclocking/vidcard/159
use Update #2
download GPU-Z, ATI WinFlash and RBE
start up GPU-Z, click the button beside the BIOS Version to get your BOIS (Back it up)
run RBE, load your 6950 BIOS, click on the 'Additional Features' Button, here you'll find an option called 'unlock 6950 -> 6970'
change your shade count, save your bios and run ATI Winflash
Works great....maybe this will help