Without azimuth info, you better have a very flexible neck, not to mention an aircraft with 360 degree vision
It doesn't take many flights under a radar information service to realise that without azimuth, most warnings will be useless. Even with an RIS one cannot spot most of the traffic mentioned if the visibility conditions are less than great.
On top of that, most PPL flights have no transponder, or have it switched off, often deliberately. This means that you have to maintain practically the same lookout as before.
Personally I would save the money and put it towards a really good top-end GPS, with an external aerial - suction cup job if you cannot get one screwed down on the roof.