I thought about that, but the initial cost would be significantly higher. I don't think it would be worth the energy savings - I only plan on staying here a few years. If the main heat pump ever needed to be replaced, I would definitely go with mini splits.
One of the main reasons I'm thinking about the window unit is how totally dirt cheap they are in the spring time. And I already have several of them piled up in the shed...
I do plan on remodeling the master bedroom and making it a bit larger with it's own separate heating/cooling, I think I probably will use a mini split there. I kinda want to play with the window unit as a proof of concept.
I'm also a bit concerned about power outages, I'm in a very rural area and it would be death to go without A/C for very long in the summer. If I just ran the one 5k unit it would be really easy to run from a very small generator. Wouldn't totally cool the house but definitely would help humidity and make things bearable.
29 SEER is pretty freakin' amazing though!
I think my current heat pump is 15 SEER...does that mean a newer mini split would cost about half to operate? Currently about 75% of my electric bill goes to the heat pump.