Vertical ref can tell you what it's like at 23,000'++, all I have been to is 17,000 in a Huey, the blades going Wok..swish...wok..swish... Minimum speed was about 40kt, Vne was around 50kt so the envelope was substantially reduced. Then when we rolled the throttle off, we had to hold a large amount of collective in to keep the rotor RPM in limits.