Please educate me - why not? Isn't the efficiency in the compressor
or even completely?
The efficiency is in a combination of ingredients. The compressor
contributes, but so does the amount of power consumed by the indoor
blower and outdoor fan motors. The indoor coil is one of two heat
exchangers, if it does not have the correct surface areas on both the
Freon and air sides, the efficiency will not be as high as it should.
Also, when you change just the outdoor unit, the refrigerant metering
device will not match the new outdoor unit, which is important. The
metering device is picked to work correctly with the exact match
between the indoor unit and outdoor unit. Then there is the indoor air
flow. It should be checked and verified to be around 400 CFM per ton.
This does not mean holding your hand over the grilles. It means using
accurate test instruments. If the air flow is low, the heat transfer
will not be there. Try driving your car with part of the radiator
blocked with cardboard all summer. (The engine may overheat) Finally,
the refrigerant charge has to be correct.
So there is a lot more involved than just changing the box that is