I am able to get 296MH/s with my HIS 6870 (reference design):
Phoenix 1.5 using phatk
worksize 128
aggression 11
bfi_int
vectors
Core at 990mHz
Mem at 880mHz
Stock voltages.
Fan at 65%
GPU ~74C with case closed.
I have been able to get 305MH/s by pushing my core to 1,000 but it is not reliable for more than a few minutes. 990mHz is my limit.