Having different bios in sli should not have any impact on performance.
Flashing the bios in a video card is pretty easy. I have used nbitor to flash a 470 bios onto a 465, I used to adjust the bios of all kinds of cards from 8800gt up to 470s. Never messed with anything newer than the 470 though.
Take a look at the attached link for a guide.http://www.mvktech.net/content/view/2069/37/
Keep in mind you can never really brick a video card as long as you have another working card, you can always re-flash the bricked one. All you would need is a working bios and another card plugged into the monitor so you can see what your doing. Of course you could flash a crazy overclock and burn the thing out, but that different.
I keep an old, like 1995, pci video card around. If the flash goes wrong I just stick in the PCI card and boot with the screen plugged into that. Then just reflash.