I'm real late to this party/knife fight and couldn't read everything BUT I do have a 12 channel hand held GPS and have been clocked by radar gun.
I am skeptical of the 80mph in normal clothing. In a downhill suit no question it's doable. Downhill suits meet FIS standards of air permeability to help prevent fallen racers from sliding too fast or too far. Speed suits do not have the permeability and are MUCH faster. I used to train with friends who speed skied and went to Worlds at Silverton and Les Arc. The local cops brought out the radar one day and clocked one guy at 103mph! Big pitch 240cm speed skis and rubber speed suit and aero helmet. Some of us ran in downhill suits and you hear the difference. The FIS suits made a loud hissing the speed suits were a quiet whoosh. The aero dynamics of clothing are a big factor even with different skin tight suits let alone baggy clothing.
For GPS you first need a good configuration of at least 4 satellites for accurate 3D. For accurate rapid descent you would need at least one low on the horizon close to right angles from direction of travel. That is often hard to do in the mountains. It may be there at the top but you could loose it part way down. Aviation applications will give more accurate readings as the horizon line is lower compared to the plane and that makes that critical low and right angled sat much more likely to be read consistently.
It'll be a fun day however it comes out and I'm plenty keen to be proven wrong. I'd certainly be in for it if I was in the area.
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