But does it match whole-system performance with their ductless H2i units? (With a variable speed air handler it might come close.)

No R410A refrigerant system is going to have a COP of even 2.0 below 0F, but Mitsubishi keeps extending the low-temp operating limit. Most of the ductless units can still put out ~70% of their rated heat output at -13F, but it's barely breaking even (COP=1.1 or so, at mid compressor speed) at those temps. To be competitive with an 85% oil boiler at New England oil & electricity prices you need to be running a COP of 1.5-2.