I can't seem to adjust the voltage as this is a non-reference design. I was able to reach 940-950 Mhz core clock stable with MSI Afterburner though. I also checked that many people use the Asus ATI Flash, but this will not apply to my card as it's once again, non reference.
I was reading on radeon bios editor and wondered if this will help me by softmodding my volt. I haven't tried it yet so I'm asking here if anyone has experienced OC'ing a non-reference 5870.