The newer BIOS supports a new RAM type. Some cards that came with the new BIOS will not work with the old BIOS; vice-versa also seems likely, although I have not observed this myself.
Well after much search and reading I finaly had the courage to flash my 015.043.000.001.000000 to the older one VER015.042.000.000.000000 but had instant crash entering windows, blue screen.
Also gave a try to the tweaked low voltage Stilt one 's and it boots to win but keep getting driver errors even after a clean re-install of the 13.12 drivers.
Tho I observed something strange, one of them had a difference when using hawaii info, RA1: F8010005
(this card have a more orange color in the cooler) , the other one have memory RA1: FA000005 ( yelowish color in the cooler). They are only five numbers appart in the serial number on the back of them so how can they have different memory numbers and colors?
By the way the one that have the RA1: FA0000005 can mine at 1040mhz core clock and 1500mhz memory but the other one RA1: F8010005 can only do 965mhz core clock and 1250 memory clock.
Like BlockChasers suggested, getting a 50mv up on trixx on the card that performs worst managed to get her to the 1020clock 1500mem with no issues but the temperature is much higher (about 6 degrees), can 't deal with that.
Any suggestions? An explanation why two cards the same model buyed in the same day came with different RA1?
What's the RA1 on yours?