the Q400 can do 285kts in the descent, but has to slow down below FL80 due to the windscreen limiting speed which affects Vmo.
I do believe that the carrier that you are talking about have been told to maintain 250kts as some maths guru has decided that it will save XXX £'s over the next 12 months.
The one thing that the Q400 doesn't do is 'run out of puff!'. It's a quick machine for a turoprop, and I used to love flying it....
