Every retail bios I have tried was the exact same core voltage on my cards. And I've tried at least a dozen of them. Every retail bios has a table that looks at the asic score and picks it's default bios based on the asic score.
It worked exactly the same on the 7900 series cards. When you modified your bios to change the voltage you did it based on what your cards asic was.
With the 290 series you cannot edit the bios or the card will not function. The bios has to be signed by amd. The only way you are going to get a lower voltage is if you run one of thestilts mining bioses that are terrible for gaming because rops are disabled, you use software to modify it from within Windows, or you create a custom bios then get amd to sign it for you like thestilt did.
But if you would like to try bioses I haven't tried, you can exclude the sapphire bioses, the msi stock and gaming bioses, the Asus bioses, the powercolor stock and pcs+ bioses, and the stock his bios. I've tried all of them just to see if any were more or less stable, and they all load my two separate brand cards to the same core voltage. Because default voltage is based on asic score.