In the stock CE system the "HD" link is stronger than the rc signal. So when the RC is boosted to FCC, the range will increase until the HD signal in CE mode becomes unusable. This might indeed double the range.
The question is: when a flightleg is flown in CE mode and HD is observed to drop to 1-2 bars, when flying the same route in FCC mode, is the HD link proven to be stronger (3-4 bars)?
In my tests the HD remained unchanged.
Im on android.