More or less heartwood does not seem to make a difference in the making of bows in the hickory here.
Could be the same routine with hard elms also.Although this type elm does not have much heartwood at all.Mostly all sapwood.Red elm has a lot more heartwood and more early wood in it.
I know from making split heartwood arrow shafts and sapwood arrow shafts from hickory there was no difference in the spine and mass.
All wood was air dried and acclimated at the same humidity of course.