Yes, the FathomX has nice performances !
I'm currently trying to qualify it for a tether length of 1000 to 1500m.
But there are precautions to take to make such tests
The cable must be unwound
( try to disconnect one of the wire when testing with a wound cable, you'll see that the link will still work ! )
Performances are differents in air and in water.
With the above telecom pair, once immersed (in fresh water !), the maximum length fall down to 290m, with a bitrate around 24 Mbit/s
I'm sure that with a cable with an outer jacket ( like BR tether )result should be far better, but this was intentional to test this "worst case".
It would be nice to have informations about attenuation vs frequency for the BR tether.
Rusty, do you have such a graph ?