To tack an extra question onto those posed by the OP...
If anybody has seen one of these devices in the flesh, what does the supplied antenna look like? I've not been able to find a picture on GRT's website.
In our case I installed a Pathfinder ADSB receiver. While it's a good unit its weakness is that it is single-frequency only, 978 UAT. With this being the case I'm missing out on all the 1090ES traffic. (I'm in Canada so yes, I'm missing the 1090ES traffic thanks to there being no terrestrial ADSB infrastructure here.)
I like the idea of the GRT product, particularly as I have a GRT HX, Mini-X and Sport EX in the panel. Having a GRT branded product would lead me to believe there should be solid compatibility between their ADSB receiver and their EFIS devices.
I'm happy to learn from the experiences of others so please do share your knowledge of the GRT Discovery device.