The KE number is (and I'm trying to say this as quietly as possible in a forum) relatively useless in regards to penetration. It is a directionless number in physics that is a sum of all of the energy imparted into the arrow from the bow. These energies are things like vibration, oscillation, heat, and momentum. The momentum part is the factor that accounts for penetration because it is given a direction vector. KE, however is a good starting point for choosing between two bows that you can shoot well and feel good in your hand; the higher the KE, generally, the higher the momentum that will be imparted into the arrow from the bow, especially when you can tune the bow well to remove oscillations and vibrations from the arrow.