Since you have downlink... how about flying a "Flight Log" capable RX,
And watching the number of antenna/frame losses? You could easily see when you are hitting the limit of range before any problems occur.
You can verify range for all of us! (For you site/aircraft/conditions, of course)
You'd need the Flight Log (or an Eagle Tree that plugs into the same spot, or similar) and an AR9000 or AR7000 with the new firmware.