William: You are correct, a carbon-fiber or even a fiberglass prod will shoot faster than steel. Steel prods are very durable, but they are heavy for the amount of power produced, hence they are slower than lightweight modern materials. You will also find that bowstrings made of modern fibers, like fastflite will give you better performance than flax or hemp, or even Dacron b50. The downside of very hard, non-stretchy strings is that they reduce the working life of the prod, since any energy that doesn't go into the bolt gets re-absorbed by the prod, rather than the string. The constant shock eventually damages the prod... by delamination or fatigue, or what have you... depending on the material.
Medieval bolts almost always have flat butts without any sort of nock or cap. Sometimes a slight hollow in the butt can correct a misfire/string-hop problem, but if so, it probably means your string is flying too high, over the bolt, or getting under it.
Going to a two-lug release rather than a single-lug in the center will allow your string to push the bolt from the start, rather than striking it. If possible, you'll want the string to push the bolt at its center, neither high nor low (center of string pushes center of bolt). You will note that roller-nut medieval bows are set up to make this fairly easy. Yes indeed, there's wisdom to be gained from looking how the professionals did it for a thousand years.
Keep up the good work, it's great hearing about others' experiments for a change, rather than having to manage all the disasters myself. Geezer