Mini-split vs gas boiler for heating
The thing is: heat pumps move heat around rather than creating it, so they are more than 100% efficient. They collect heat from the outside air and move it inside, and depending on the available heat outside, the output inside can be several times greater than if that same energy were to be used to generate heat (as in a resistive space heater).

With your numbers, and assuming your boiler is 80% efficient, you only need the heat pump to be about 170% efficient to be a net win — that is, a COP of 1.7 or greater. This is easy for a heat pump to achieve, even in cold weather. 

In this chat discussion Shalvenay pointed me to a chart for Mitsubishi heat pumps, and the MSZ-GL12NA-U1 has a COP of 3.84 when the outdoor temperature is 47° F and 3.13 when it's down to 17°. Assuming your prices are as you say, this is a definite win for the heat pump.

But are you sure you're factoring in the full electricity costs? I know it's a year later, but looking at my Eversource bill, I see $0.11397 for Generation Service Charge, but there are bunch of per-kWh Delivery charges as well, coming out to about $0.223 total per kWh — or $6.54/therm. That makes the math look somewhat different — you'd need a COP of 3.4 or greater for the heat pump to be a win over gas priced at $1.50/therm. According to the data sheet, your units should still hit that when it's relatively warm out, but won't as it gets cooler.

There is also another factor to consider. Burning natural gas is relatively clean as fossil fuels go, but still releases CO₂ — about 12 lbs per therm burned. In Massachusetts, about .9 lbs are released per kWh of electricity generated — about 25.7 lbs per therm. If we again go with 80% efficiency for the boiler, that means a COP of 1.7 or better is a win for carbon emissions, even if not for your pocketbook.