All the rebound you need is naturally contained within the elasticity of your muscles and tendons. Any artificial form will only decrease efficiency. All the so called rebound materials and gimmicks are energy vampires. They consume more energy than they "return". This is already occurring inside your muscles, why would you want to add another system in place to go through another energy transformation to lose more efficiency? Any material contained in the soles of VFFs is there for protection and not for "springback". Vibram understands this. Focus more on running form and not so much on gimmicky band aids. There are no perpetual motion machines. Anytime you add another system, you will always lose efficiency.
Sounds a bit dogmatic to me. You are not necessarily adding another energy transformation, just a longer compression and rebound.
The calf only rebounds in the neighborhood of 30% of the energy from it's stretching motion. A mechanical spring or elastic material could be far more efficient.
Actually, at "ideal" pace, the calf and achilles will rebound and reuse 50% of the energy it absorbs. Any material that stands between the foot and the surface is creating a shock absorption buffer zone that reduces the energy transmission. Springs and elastic are terrible when it comes to the conservation and reuse of energy, that is why shock absorbers are made of these materials. A spring has a home position, the shape of the spring in rest, ultimately the spring wants to return to equilibrium.
For your achilles to optimize energy return there should be no additional layers of absorption. I imagine, like many other parts of human anatomy, that the achilles under ideal situations (pure force imparted without the loss due to skin, muscles, and joints) that it would be able to return greater than 50%, but it is limited by other structures.