I followed a guide on how to flash with NVflash in Windows, i have flashed my card dozens of times seeing the differences in bios's, mostly voltage differences..
The 580 bios was an experiment, 480 was 100% stable as long as i left the clocks alone, but even with voltage boost, the 480 would crash under the 580 bios, i would say it is probably different memory being used on the 580 that may cause it.
Here is a 3Dmark run with the 580 bios: http://www.3dmark.com/3dm11/6039965
To flash the 580 bios, i had to bypass all hardware ID's on the 480, which NVflash can do, i do not recommend anyone to try it however.
For a guide on how to flash NV cards:
0) Be very, very, very sure you have the right bios, and only one GPU in the system.
1) Put the new bios file in the same directory as nvflash. (I like c:/nvflash, easy to get to)
2) Fire up the command line, get to the nvflash directory.
3) Type "nvflash -4 -5 -6 biosfilename.rom", no quotes.
4) Hit enter.
5) If it gives you nonsense about a protected bios, type nvflash /? and find the flag for unlocking the bios, use it, then go to #3.
This is from overclockers.com, and has never failed me ever.