Thanks for you kind words Jason!
I have no experience with the RXP GTNs, but reading the manual I assume that you can use the pop up mode, and the Avanti will follow the flight plan, if as NAV/BRG source has been selected the FMS.
What I don't now is how interacts with X-Plane internal flight plan mechanism, cause I am interrogating it to extract some data. If writes the waypoints into X-Plane's flight plan, I believe it will be ok, otherwise (use an external method) might not be compatible.
In any case, I am not suggesting to purchase the GTN750 just to use it with the Avanti. If you do purchase it for another aircraft, then we can see if can be used in the Avanti. If requires any kind of small adaptation, might be possible to work it out, but won't be "integrated" into the Avanti.
My target is with every update to get closer to the simulation of Rockwell/Collins Proline 21 avionics suite, as used in the Avanti.