Steve,
2% efficiency is all we need! We're not talking about browsing Facebook or making phone calls. We're talking about a 500 byte UDP packet every 5 seconds, not chatty bandwidth hungry TCP.
Here's a good example: https://tx.ozrunways.com/tracks?flight=7229f85e-840c-4f56-a6c0-2c8b2ae974a0&type=kml (open in Google Earth) A flight from Home Valley Station in WA to Halls Creek by one of our staff. Not a highway in sight and no cell coverage on the ground, but a few packets as soon as we're airborne. Then a short gap, then almost continuous coverage all the way to Hall's Creek no higher than 5,000. He actively avoided another aircraft crossing his path within a mile or two and 500 feet. Then used it to coordinate with other scenic aircraft over the Bungle Bungles.
Here's another good one: Bourke to Old Station at 9,000. A few minute long outages is all there is. https://tx.ozrunways.com/tracks?flight=787979c6-0a4a-4391-bdf6-fb7d7cee5d95&type=kml
Near towns where aircraft actually converge, coverage is 100%.
Telstra has no idea how good their network actually is from a few thousand feet up. Probably because they never actually tested it that way! :)
The proof is in the data. We have that in 411,824 logged flights as of this moment. I suggest you go fly and try it our for yourself!