I found this out AFTER my install was complete! I was testing my G3X with GI275 backup after the installation noticing that Baro was not synchronizing between the two. After calling my Avionics guy about it (as he had assured me "The GI275 does everything the G5 does, but better" ) he tells me "Oh, it turns out I was wrong there. . the GI275 can't talk to the G3x, or drive the auto-pilot (GFC500).
Further researched confirmed it basically has to do with that fact that GI275 is not certified to talk CanBus, which is what is needed for it to talk to the G3x, the GFC500, and the GTN. So is basically just an expensive and 'dumb' ADI. The G5 can talk Canbus, so it can play nice with my other equipment.
Soo. . flash forward a few months and I am talking to my NEW Avionics guy about swapping out the GI275 with a G5. The good news is that he just got back to me today and said he spoke with Garmin and they are promising the GI275 will be certified for Canbus "Within 12 months". So now I have to decide if I 'wait it out', or perform the G5 swap.