At full speed, with the same voltage and clock rates your card will use the same amount of power as before. There might be a tiny gain in performance, so efficiency is slightly better.
In idle, you can achieve the same with a 480 BIOS by activating, sorry i forgot the exact name of this option, some power save mode in the driver. The 580s don't have that option, it is activated by default, and you can't change that.
Memory timings should improve efficiency, but you also have more power consumption after all, because GPU utilization at least for the mem controller is also raised.
The two BIOS files for the XFX GTR should also fit your card. Though, you should consider the safety suggestions if you have some special edition; so be sure you have samsung vram and an IR3567B voltage controller on your card.