Sounds about right. And needle valves can slow things down too.
Heh...
Even if it's inefficient, it may be sufficient. How much power does it really take to spin that little steel ring?
OK.
One quick test would be a cheap electret microphone near the bearing, the whole affair being down under a pile of sandbags for safety, to convert the siren tone into an electrical signal one can display on an oscilloscope and measure the frequency. Knowing that the siren tone is ultrasonic would very much constrain the problem.
Joe Gwinn