Hey Isaac
I noticed this question spurs interest in the communitiy. But on the other hand, I'm afraid it won't be answered here, since I simply don't feel I can make a bow as good as to say it was the woods fault if it shoots poorly.
However, I will set aside a piece of that sapwood, let it season thoroughly, and when I feel I know what I'm doing, I shall make two bows of the same design, one heart- and one sapwood. That, however, is still some time off...
Till then, someone else might have to work on your question. All I can say at the moment: the transition from sap- to heartwood looks really, really good.
Those 5 to 10 year old locust stems are readily available, since they make excellent and durable fenceposts. So give it a try.