I don't know what you mean by 'closet style' but in central Oregon, where temps get down below -10 in winter, Mitsubishi Mr Slim with hyper-heat is the way to go. It is rated to -13.

Check out the web site for "efficiency maine" for a case study.

Just wanted to mention that this is an apartment style condo with no gas lines. So I'm stuck with heat pumps, and they have to be the half in half out kind. Nothing with an outdoor split unit will work for me. And I can't poke any more holes in my wall aside from the one I already have.

I suggest you look into the Carrier Greenspeed heat pump. They are the inverter type which have better performance than the traditional air source heat pump at low temperatures..

I would think a 2 ton heat pump would be sufficient for your 1000 sq. foot condo. The data shows at 20 degrees F the heat pump can put its rated 24,000 BTUs. Below 20 it begins to drop off. At 10 degrees it is around 22,000 BTUs, at 0 degrees the heat output is about 17,000 BTUs. You probably would need back up resistance heat below 10 degrees F.

I don't think a conventional split system will work for OP's application. It seems he has a motel type HP system. HPs have come a long way in improvements. Suggest you start with mini split systems but you should be prepared for possible Bldg modifications. Keep us posted.

IMO

wouldn't this be a "package" unit?

What is the model number/size of your unit? Heatpumps are generally sized for cooling capacity and supplemented for heating (if necessary) with electric strip backup. Being as you are in Boston, though, heating capacity would be the more significant need. If your present unit isn't heating adequately, you may need a larger unit (2 ton appears to be the largest they make) but this may compromise dehumidification in the summer.