I have so much need for melting that recently I've designated a fourth magma smelter just for melting acquired items. My furnace operators are fast, but every siege and every caravan brings hundreds of items, plus I melt the items made by trainees (or even legendary - legendary in his description doesn't mean he won't make a well-crafted items).
Apart from that, my marksdwarves train best when using metal bolts, and they use huge quantities of them: I have one forge which has making bolts on repeat, plus I makes some additional batches from time to time, and I buy and "reacquire" all bolts, no matter the price. And my bone carver also makes bolts on repeat. This means that I have many good weaponsmiths, and thus many masterwork bolts. I melt all single bolts except masterwork, though even these are finally broken during practice.
I said all that just to indicate that the original idea of using bolts for making metal wouldn't work at all for me and my setup, and I see no way to change it. The coins seem more appropriate - in .31.25 a full stack of coins was yielding 1.1 bars of metal, and it was using 1.0 bars of metal to make. I don't know about the return rate in current version (I make only one stack of adamantine, steel, gold, silver and copper coins per year, as a kind of chronicle) but making a stack of coins still require 1 bar only…