I think that the evidence of the Yachtsman for the height and speed of the Chinook should be considered carefully as it may suffer from the effects of habituation and scale. He said that he was unfamiliar with the Chinook but was used to seeing SAR helicopters. The Chinook is a much larger aircraft and it would subtend a greater angle at the eye of the observer. It would therefore give the impression of being lower than its true height. This would then result its appearing to travel a shorter distance in the same time, giving the impression of flying more slowly than its true speed.
The effects of scale in the opposite sense can be seen with radio control scale model aircraft. They appear to be flying much faster than their true speed. The Red Bull air races have more dramatic impact as a result of greater apparent speed because the aircraft which are flown in them are relatively small.
As the Chinook is about twice the size of typical SAR helicopters I think the estimates by the Yachtsman of its height and speed should be increased by a factor of about two. This would mean the Chinook would have been flying at around 140 knots, which would agree with other evidence for its speed at that stage of the flight.