Expanding on the original question, what allows for some helicopters to have a much higher Vne than the example R22? I calculate the R22 rotor tip-speed at 458MPH, well below the speed of sound. (I understand that is a practical limit. I can't imagine a blade going supersonic, then subsonic, on each rev and staying together for long.) If a heli has a Vne of 150K, is it that high mainly because the rotor speed at the tip is much higher than the R22? I can see where spinning the rotor faster would give a lower AoA on the retreating blade, allowing for a higher airframe forward speed before reaching critical AoA. But, it also makes the advancing blade closer to going supersonic.
Further obfuscation: the advantage of the X2 (for example) is having a balance of lift on the two discs, so RBS on one disc is balanced by RBS on the other. (Perhaps a better way to say it is that lift losses on one disc due to RBS are compensated for by the other disc having max lift in that area.) So, at the X2's top speed, are the retreating blades stalled?