Brian,
Not to take away from the excellent performance of the H-43, but to explain why it has it, the reason is the disk loading. The Huskie was layed out when heavy piston engines were needed so the power came at a steep empty weight. This made the designers thrifty with power, and the best way was to use big rotors that needed less power. Thus, the H-43, with a 48 foot rotor disk weighs in at about 9000 pounds. The piston engine was replaced by a turbine, which could develop the transmission rating up to high altitude, but just as inportantly, it weighs about 1/3 of the weight of the piston, so all that weight savings gets to become payload.
Another example is the S-55, which has a 53 foot rotor diameter for about 8600 pounds gross weight. The Black Hawk has the same sized disk for 24000 pounds!
The lift advantage of light disk loading is awesome. The power needs of the rotor are proportional to the square root of the disk loading, so a 6.5 pound per square foot rotor (Bell 412) needs about 40% more power than a 3 pound per square foot rotor (H-43). This means that for the same power, it lifts 40% more.
In other words, the efficiency of the H-43 is not because it is a synchropter but because it has a great big rotor!
The downside of the old machines is that they also beefed up hover performance by having skinny blades with low rotor solidity (ratio of blade area to disk area). With low solidity, the blades are near optimal lift to drag in a hover, so more weight can be lifted, but the downside is that the rotor stalls at slower speeds.
The Huskie has a max speed of only about 110 knots, if I recall correctly, which is part payment for that hover efficiency.