No one has ever reported flying any airplane of any size with the DX-6 system out of visual range.
I fly many planes with it, up to Senior Kadets, and never had any problems with range.
The actual situation is the small size of the receiving antennas being blanked out from "seeing" the transmitted signal by large items in the airplane.
If this can't happen, the range of the DX-6 has been tested out to a couple of miles.