with regard to absorbing impact energy... the vast majority of energy is absorbed during plastic-deformation
This is a quite generic statement, and not entirely true for crashworthiness of aircraft. Aircraft structure, highly optimized for low weight, often fails in a buckling mode, not in overstress. Therefore the plastic deformation is restricted to a very small part of the overall structure, and the energy dissipation is low, compared to the overall volume of material.
For composite structure you can generally state, that it could be designed to have superiour crash resistance. Using monolithic, thick walled, woven fabric, hybrid material design (Carbon/Aramid, or even better Carbon/Polyethylen), gives you crashworthiness unparalleled by any metal structure. Formula 1 demonstrates this impressively.
If you want to have a structure optimized for low weight, you will use unidirectional tapes, thin walled structure and carbon fibre only. Such design typically fails without much energy dissipation.
So the 787 could be both, superior to all metal competitors with regard to impact resistance, or a total nightmare, depending on the knowledge of the boeing engineers and the ethics of boeing management. Future will tell us.
Looking at the pictures of the crash tests, I am not too convinced, that crashworthiness was the primary reason for chosing composites. Looking at the boeing advertisments also looks more light low weight and economy were the driving factors.