To answer your first question: reported efficiencies of microwave transmission systems of over 80% are not uncommon. Of course, a lot depends on the context; materials and techniques that are used, real or laboratory conditions, etc. Most importantly, these efficiencies are reported in experiments and applications in which distances are not more than a couple of kilometres. It is to be awaited whether these numbers can also be reached in applications over hundreds of thousands of kilometres.
Your second comment is actually an interesting thought. Never really considered it before. However, a quick calculation can show that it would probably never be a viable option; transporting batteries from the Moon to Earth implies that they would also need to be returned to be charged again. A very optimistic calculation for how much energy a battery can carry these days per kilogram gives 1.4E6 J. The energy required to bring one kilogram only into a low Earth orbit without accounting for friction and gravity losses is approximately 3E7 J, an order of magnitude bigger. Taking into account the additional energy for manufacturing, friction losses, launch from the Moon and energy leakages it becomes clear that even with a tremendous progress in battery technology, it would not be a viable option. Nonetheless, batteries may probably be useful in some parts of a Luna Ring system but most likely not in the energy transfer part.