This is a very interesting thread. I just got a professional install of a Daikin unit for the improved attic area of our house. It has been working well and saves $$ vs. the electric baseboard heaters that were the prior heating method. I do find that the air inlet screen of the outside unit is almost 80% frosted-over on cold (30-35F), damp days. That can't help with efficiency. I was considering removing the flat screen and replacing it with a homemade formed screen assembly with maybe 3X the surface area. Not sure that would help but I'm considering it.
I have been considering a mini-split for the shop. It is a 1000 Ft^2 x 9 Foot to ceiling. I was thinking of having a backup heat for quick heat-up or for very low outside temperatures. I have a ducted propane room heater that works well but the fan is very noisy. Maybe that could be the back-up heat. Temperatures are usually 40F or higher for most of the winter but there can be weeks at 10-30F from time to time.
Now I'm "split" on whether to go with a pre-purged system or not. I also wonder if just one inside unit will disperse the heat/cool to the far corners of the shop? Are there dual DIY inside unit systems with pre-charged lines? I know a window AC unit works great near the unit but the far end of the shop is hot so having two inside units might be better.