All 4 of my Sapphire 5830s do 307-310mhash in Ubuntu
Flashed with RBE:
Core: 1000
Mem: 344
Voltage: Stock
Fans: 85-100%
Temp:58-72C(depending on when my AC runs)
I went XFX just for their lifetime warranty. I figured as a miner, that'd be good to have. 20mh/s less is alright by me, I suppose.