I keep mine at 470MHhash/s but it could easily do your hashrate with some extra cooling. I don't want to push it too hard because I want to keep this card as long as possible. Same for my 5850.
I can do even higher cause the card is only at 34*C full load using water cooling. But I don't want to risk adding more voltage and frying the VRMs.
http://i.imgur.com/PjjbI.png
gonna try that for a day and see if it's stable
I see you use Phoenix miner instead of OpenCL, and as I know it is higher hash rate for all 58xx and 5970 as long as you stay with Phoenix. But what about temperatures? Are they the same as using OpenCL? (considering the same clocks and voltage) and how much hash rate in average could you possibly increase with single 5870 with Phoenix over OpenCL?
I found Pheonix was on average 15-20Mhash/s faster than OpenCL after an improved kernel. Temperatures had no differences.