Based on much reading on the subject - harder is not usually better when it comes to terminal performance with CB's. Harder is part of the recipe, necessary at times, to achieve higher velocity without leading, or in some cases - stripping the lands.
If your 15BHN alloy is giving good accuracy in the velocity range you want - it is a very good choice. FWIW - the 3rd Edition Lyman Cast Bullet Handbook - pg. 96 has an article by C.E. Harris & Dennis Marshal re: cast for hunting. The conclusion suggests that 16BHN is perhaps optimal. Your 15BHN is right there.
Best regards-
Sky C.