5850 voltage control

My XFX 5850 doesn't seem to have any voltage control. I tried to adjust the voltage with Afterburner and Trixx, but the voltage is locked. I can easily change the voltage on my 6950. Would voltage control be available if I make a new BIOS with Radeon BIOS editor or if I downloaded a new BIOS? My card is this one here http://xfxforce.com/en-us/products/graphiccards/hd 5000series/5850.aspx

If it's that model specifically, it's a non-reference PCB version so it's possible that XFX removed voltage controls from it. If this is the case, I should think that making a new BIOS wouldn't do anything due to the lack of programmable voltage control.