Mini -splits have come along way in recent years.With the inverter technology, mini-splits can perform quite well in efficiency.Like all heat pumps efficiency is rated in COP. The higher the COP the more efficient they become. Electric rates, climate conditions,soil conditions, square footage of building,yard size,etc. all need to be factored into for a proper comparision.

I have installed inverter driven Mitsubishi PUMY/PVFY variable indoor/outdoor units before just never mated them up to solar. If going with a vertical bore i think you would be better with the whole house Mitsubishi and PV but consider, you technically should not get the 30% on the mitsubishi system even if it is powered by solar, I know most people will not file taxes this way but legally speaking it is not correct. The downside is the Mitsubishi only comes in 3 and 4 ton models, it does not do any domestic hot water, it runs into issues of needing backup heat, while rarely needed these systems require duct heaters as the air handlers don't have room for heat strips. Lastly, it is typically the coldest days of the year that need the most heat and have the least sun. I live in portland oregon where we can get rain for 10-30 days straight with no sun, while PV will still get some power its not nearly enough.

As an HVAC contractor that does both geothermal and mini splits and whole house inverter heat pump systems, geothermal IMHO is still way beyond inverters. It can do so much more, work in more diverse situations and be pretty much the same efficiency almost regardless of outdoor temp.

I was listening to a builder’s presentation of the benefits of coupling PV with Geo. One big surprise for me was that Geo cut the PV pay off time by two thirds.
This was a commercial / school builder and the example was a school. I have not seen data for residential homes to see if it tracks the same for residential or not.

SCIP i would imagine its the same, with geothermal you are already off setting costs by 40-70% depending on what the previous heat source was. If solar cuts your heating costs by 30% then yes it cuts the payback. This should be fairly true for any setting. I personally am not a big fan of solar. Solar panels from what I have heard have a life of 20 years vs a geothermal heat pump unit at 30 years and the piping at 55+ years. Factoring in replacement costs of the home(not always the owners time at the home) and geothermal is a better source IMHO, but then again I might be a bit biased as I do install a sell ground source heat pumps.

what would be the disadvantages of installing one city multi outside with three of there pvfy inside? Versus having to put three traditional heat pumps outside with other brands? (carrier greenspeed?)

I have yet to see a pay back work out on a geo system when compared to building a passive house from the start. Reducing the HVAC load by 90% passively usually cost a similar amount to a GEO system At that point there is no payback in a GEO. Build the shell right, no long term cost in passive.

Brian