I mean, a simple way in which you can use feruchemy to make allomancy more powerful is using pewter. Since, both of the powers of pewter are so similar, we have evidence that instead of storing 'your muscles' in pewter, you can store the pure strength you get while burning pewter. Such, a metalmind would likely work like a stronger version of normal A-pewter, giving much much more power, which you again store in a metalmind. We have some evidence that this is what the Lord Ruler was doing, since he didn't get lots and lots of muscles while he increased his strength. (That is what should happen if he was doing normal compounding)