I'm late to the party, but FWIW, I use a 18k btu Fujitsu Halcyon mini-split that has a built-in low ambient feature that allows me to run in the winter time down to something like -14 degrees. It's a very nice unit and I'd recommend Fujitsu being right up there with Mitsubishi's..
This would be nice especially where I'm located so far north. I had a tech quote me for 10 Tons of units one of them was either a Fujitsu or LG that he said was made for gardens and he said the Mitsubishi would be around the same range. He wanted 18.5 k to do it though and that was just out of my range. I'm gonna rock this Mr cool unit for a bit and see how it does and if it holds up okay I'll add some more of those for other rooms. I can air cool most of the year anyways so I'll start veg soon for my other flower room and by the time they are ready night temps will be low enough to air cool til I figure it out. Unless I hit bigger than expected numbers for current run in that case I'll buy another split system sooner.
To deal with crazy winter temps here I built a rather large stand with a roof that gets the units up off the ground because we regularly have like 4 ft of snow on the ground. If the units start to get too cold the shelter is big enough that I can add side panels to block wind and keep some of the warm air in there. If need be I could run my veg exhaust into the shelter to provide some heat as well. My unit says it will work down to 5 degrees, but it does sometimes get colder than that here. I have heard a lot of guys up here say that they build similar structures, mine is just a bit over kill for the one unit but I made it that way so I could hopefully fit 3-4 units when I eventually get the money.
Although, I have been thinking of just cramming as much supplemental lighting as I can into the rooms for winter and saying f*** the c02 and just air cool. Just cause I could add so many watts and still air cool the room. My winter's are pretty long here and I feel that unless you are maxed out on available light that adding more light ups the yield more than c02 would.