It's been stated throughout the series that the feruchemy comes from the body of the feruchemist.
For instance; In order to store better sight, you have to first spend some time with worse sight than usual.
And yet, I can't help but wonder if you could potentially try it another way around.
Why store positive attributes which force your body into a weakened state, when you could store negative attributes which force your body into a strengthened state.
Then you could just constantly store attributes and be more powerful all the time and not just when you you use the attributes.
The only potential downside to that idea, were it to be possible, is that after you eventually reach a point where you can no longer store more of an attribute in a metal, then it becomes useless; and you have to buy more metal.
But the, I guess, you could just sell it to allomancers and buy more metal with the money they give you.
Since they cannot reach the attributes that someone else stored within, the metal just becomes average to them, doesn't it?