Hey Bob,
I have a bit of experience with installing GI-275’s. I actually installed 3 GI-275s in a Mooney M20C. Picture attached. The GI-275s are great units and have a ton of capabilities. They work together in a way similar to a flight deck. I would recommend going with at least the GI-275 ADAHRS + AP and GI-275 ADAHRS. If they are installed in accordance with the STC install manual with a reversionary switch they can become primary IFR instrumentation for attitude, heading, airspeed, altitude, vertical speed, and turn information. Theoretically, if so desired the other instruments could be removed.(Airspeed, Altimeter, VSI, Turn Coordinator)
The interface to the KFC225 is pretty extensive. In order to comply with the STC the autopilot must be connected to the primary ADI which will be the GI-275 ADAHRS + AP. This is the only version of the GI-275 that will work with third party autopilots. I don’t see any way that the other GI275 could drive the KFC 225 if the GI 275 + AP is turned off.
Garmin is also coming out with the capability of the GI-275s to be able to drive the GFC 500. So that would still be an option if needed.
One thing to think about when removing the KCS 55A system(KI-256 and KI-525) is that you will most probably be getting rid of synchro heading. Instruments that you have that were receiving that synchro heading before will no longer be receiving it. Examples of units that might be receiving synchro heading would be Shadin Fuel Air Data computers, Stormscope, Skywatch, Radar etc...
These are just a few thoughts.
Isaac Maxson
Private Pilot | Avionics Technician