Hi Richard,
I had a 'look' at this one (on paper) a while back. It looks like it should work fine, but is pretty dear for what it is, though probably par for the course for anything with 'Flarm' associations. NavBoys certainly seem a good outfit to deal with in my experience, so I certainly don't think they are trying to rip anyone off.
As I have said before, I started out with the standard short antenna that came with the FlarmMouse - which is extremely position critical, so changed it for a PAW end-fed dipole as supplied with the Classic Kit and got my best range (7Km) in testing with this setup. I also tried a PAW 'centre fed' dipole, located inside the left knee hole of my pod. Whilst the signal was sound to my port side, reception to starboard was significantly attenuated by my legs, the aircraft battery, assorted wiring, metalwork, etc. I think the PAW dipole would however work fine if it can be mounted up high - for example on the right hand side of the aircraft front screen or front edge of one of the side windows (which I don't of course have). If a choice has to be made, it is of course best to keep any antennas facing towards the front of the aircraft to ensure the best reception is directed towards 'head-on' contacts.
I am hoping to reshuffle my tablet and other equipment this weekend to re-site the FlarmMouse to the extreme right side of my coaming, so I can position the Flarm (P3i) end fed dipole out to the extreme right of the screen well clear of obstructions. I will advise on any improvement.
Regards
Peter