In the Q400, the Vmo speeds are as follows:
0 - 8000ft: 245 KIAS
10,000ft: 282 KIAS
18,000ft: 286 KIAS
20,000ft: 275 KIAS
25,000ft: 248 KIAS
Keep in mind that the Q400's high and low "Airspeed cues" were just barely certifiable by the FAA. It constantly is moving up and down, espically below 10,000ft.