Olive Oil
<<I believe the BOI accepted that this helicopter was cruising at an IAS in excess of 140 knots>>
I suggest you read Boeing's "Analysis of Available Data" (author Mitchel) available on the web.
It is the only decent analysis done by any authority.
It has that over the bulk of the flight (say, up to the position of waypoint change) the average air speed was at the full cruise speed of 135 kts - from waypoint change to impact, the calculated actual air speed would have been 120-125 kts.
There has always been a spin to suggest that the pilots were pushing it - read the early transcripts of the early inquiries.
To understand this crash you have to do the chartwork and distance/time analysis strictly.
Further, the Boeing analysis fits with the AAIB's conclusion that the SuperTANS had retrospectively been accurate over the critical phase and so the position at which the waypoint was changed can be relied upon and so is an important reference and therefore we can look at this position on a detailed chart and realise that it was very close in.
I say so close in that they would not have relied upon SuperTANS or DR or (difficult) visual judgment to have got that close in at their speed - that is why I believe they were referring to something else that ultimately misled them.