Your right on the money here, the problem IS the lack of energy, the inability to fire the projectile at a higher speed,
If for instance you fire a 10.3 gr pellet at 700fps into your test medium you will get particular sized hole (the shock radius, the distance the material is displaced) and it takes a percentage of it's energy with it as it penetrates through.
Now fire the same pellet at 900fps and you get a much larger hole yet the pellet takes even more of it's energy with it, it's the same pellet but the impact speed is the defining factor for the size of the cavity.
So because we cant use the 900fps, as it would be over 12ftlbs, all we can do is use a lighter pellet to create a greater shock radius or cavity whilst limiting the amount of energy wasted by over penetration.
But this simply does not matter unless you put your shot in the right place, and that's where the lighter faster pellet is more forgiving.