Using walnut lams definitely creates a limb with lighter mass weight and gives the bow potential for higher performance….. but…. This wood can vary dramatically in deflection strength depending on grain configuration and density. I have, and I use a lot of black walnut, and there is a lot of difference between flat wild grain material and straight vertical gran stuff. Branch wood vs trunk milled out of the same tree can give you totally different characteristics in density, deflection, and even compression.
1/4 sawn vertical grain walnut lams and wedges with tight grain can make great core materials.