As I understand it now, the energy of the electron/positron together will contain whatever energy is leeched from the laser. Correct me if I'm wrong here. That means that what inefficiency there is will be caused by the focusing and recycling mechanism and any other outside apparatus, in addition to heat loss from whatever electricity is needed, inefficiency in producing the energy for the laser, and whatever material requirements there are.
I don't doubt that's simplifying things. But if you consider the massive amounts of energy we put into mining coal, oil, uranium, etc (including shipping, fuel and manpower for extraction equipment, sorting, refinement, and facilities operation) it could be up there with flywheels in vacuum in terms of efficient energy storage if we can make the laser and confinement devices cheap enough and efficient enough to operate. And it would be a fantastic financial resource for space-based facilities - just construct a huge solar collector, use the energy to create antimatter, use a lot less energy than what would be required in full gravity to make magnetic containment for it, and sell what you don't need.
There's just not enough information to satisfy me on this. Reading the
original paper, it sounds like it was originally related to one of Bohr's conjectures on a maximum limitation to how strong a laser/QED field can be, which is fascinating by itself, but I admit I couldn't begin to pick apart how efficient this would be.