Some "madeleine moments" here. As one who trained on (RAF!) Sioux I'm sure the yellow line was an horizon. Not so much artificial but (as BEags will no doubt remember from UAS days) as a datum for visual flying.
IIRC the Vne was 91 kts (is that a round figure in mph?) and could indeed be only achieved "downhill". But the RAF Sioux were supercharged, I don't think the Hairy Arm Corps ones were.
Thinking (BEags again) of the Beaver and its use for jollies, I remember one instance when P*rc* S*tt*n was FATOC-ing at Lisburn, and was keen to get a buckshee trip back to ODIZ.
The plan went thusly:
1. Invent and task totally bogus VIP task for Beaver to pick up pax from ODIZ (always handy for Aldershot).
2. Add himself to manifest for "empty" outbound leg.
3. Cancel task at last minute, getting the (expected) response - "Bugger - after all that planning! We'll go anyway as a training trip!"
4. Result.
[Edited to add: 91 kts = 105mph and the cruise IIRC was 74 kts = 85mph, so that may have been the reason .....
]