Ok, so I went out and did a range test today in a location where my previous best range has been 999ft (or 300m) for control, with intermittent video - but often RTH kicked in while video feed was fine.
Having modified the tx and range extender to accept the Horizon FPV "Blackhawk" antennas for 5.8 and 2.4 respectively the test today was conducted with a fully charged range extender, and Energizer Ultimate Lithium batteries in the Tx that have done about 7 or 8 previous flights. I was flying such that the antennas were always line of sight with the aircraft, but did fly over trees at one point when heading east towards the river, as you'll see from the Flytrex map below.
Here's a Flytrex mission profile:
http://www.flytrex.com/mission/quadcopt ... ton-16378/ If you look through my previous missions you can find ones in this same area and the max distances I was able to fly.
Here's a screen shot showing perfectly clear video and the range with the aircraft facing fully away from me, i.e. not yawing to present max antenna profile:
https://drive.google.com/file/d/0B9VWpq ... sp=sharing
I found that the antennas were not massively directional, and you didn't have to be too careful to point directly at the aircraft. The tx didn't feel too top heavy and I'm particularly impressed by the difference such a tiny 5.8 antenna can make. This thing is no bigger than a 35mm film canister.
So in summary, just adding the Horizon FPC antennas to tx and range extender and doing absolutely nothing to the aircraft I have doubled my effective control range, and increased FPV range by 50%. By yawing the aircraft somewhat to present one wifi antenna cleanly I was able to get FPV out to 1800ft. By adding the reciprocal Horizon antennas onto the airframe I would imagine you would be looking at effective ranges for both that are going to push out further towards 3k?!
If I get chance later I'm going to edit together a video on how I did the mods to accept the Horizon antennas (or any, really) for those who might want to do something like this but are as inexperienced at the hands-on modding side of things as I am!