Overclock on sapphire 5850 non ref.

[vertical777]

hi I'm planning on getting this card since the reference card is not available:

and I'm looking into overclocking it to match at least the 5870's power, I found almost all overclockers had no problems in overclocking these cards.
the problem is: this is NOT a reference PCB and thus has no over voltage feature. I'm wondering if I can flash it with the BIOS of a sapphire 5870 vaporx
which (I believe) has the same kind of PCB. here's a link to someone that was sucessful in flashing the non ref. card:
find him here (iddle of the page)
he's pointing to this BIOS:
5870 vaporx bios
looking at his system specs he OC'ed it to 925/1225 (dunno if that's with the flashed bios or not)
I need your opinions on this.

BUT if I can get the 5870 performance without flashing and working with only the STOCK voltages then that would be better

Any other methods/tips to achieve my goal would be much appreciated, thanks
I want a performance similar to the OC'ed 5850 in this article which comes close to the 5870

 

[treefrog07]

It's a gamble. If the chip had the 5870's capability, sapphire would have installed in on the 5870 pcb and sold it for the higher price.

 

[vertical777]

but still, in the OCC article they achieved 870/1225 clock with stock volts (I assume they are on stock volts since they did not indicate a votage increase) on a reference card. and if I get the luck of the draw (seeing as even two completely Identical cards overclock differently) I might match those figures with a non reference card.