The Sikorsky 4th gen blade has an advanced airfoil that is quite efficient, and a modified anhedral tip that is a bit better than that on the S92. These were designed with very advanced CFD design tools. The combination makes the 4th gen work. Certainly the solidity is chosen to get close to peak figure of merit at the normal takeoff hover condition, but with enough extra solidity (which reduces hover efficiency) to push off stall at normal cruise speed.
As I described a long time ago, the BERP blade on the EH-101 costs it not quite 2,000 lbs of lift relative to the 3rd gen blades on the S92. I do not know how efficient the R&D BERP blade is, but the regular BERP blade is about 1987 technology, perhaps 3.5% less efficient that the S-92 and about 2% less efficient than the blades on the EC-225.