The Barberpole on the 1900 B/C is not fixed it will comeback to around 205 kts @ FL250 and you will be able to fly 247 when descending through FL 160 give or take some FL's.
With regard to the barberpole there are more things affecting a plane than the Mcrit, coffin corner etc. What about limits as gust factors Va,
Vb,Vc from what I recall these are all based on TAS. So theoretically you could fly way to fast at altitude than what the plane was build for. That's why the barberpole comes down.
IMHO