hmmm well just off the top of my head i would say that metallic purity could perhaps be a factor in the exchange ratio of storing an ability. We know that in allomancy the metal's purity--and ratio when mixed into an alloy--is important. Incorrect mixtures don't work right and can make the user sick when burning. It's not strictly said in the books but perhaps you could extrapolate from this that the purer / better mixed the metal is the better the exchange ratio. And perhaps this could be true for all metallic magics.
Although I tend to think that really all Feruchemy is a 1:1, and would prefer that it stays that way in the rules... in the interest of game balance something like that might be possible.