Seldom
Perhaps you have not read the findings of the Investigating BOI - they were quite specific that the forecast weather would have required flight in accordance with IFR in the vicinity of the Mull. The actual observed weather at the Mull was considerably worse than the forecast weather - yet the aircraft average groundspeed from the radar fix on leaving Belfast CTZ until impact was 158 kts. In this respect you may recall that the AAIB assessment of the aircraft speed at impact was "approx 150 kts" whilst attempting an escape manouvre.
One other interesting point is that if the Chinook did slow down to an IAS of between "60-80 kts" (as described by the Yachtsman) then it would have had to have exceeded Vne at some stage of the transit in order to have achieved the known parameters prior to impact.