Hey Carrianne,
Perhaps you better should have 
When power is cut off from stepper motors they loose their Electromagnetic Detent so they can hold their position only by the remaining Magnetic moment, which is usually not enough to prevent massive moving parts in the kinetic chain from moving them by gravity.
The remedy for this is not buying a beefier stepper but to switch the power on on the CNC Controller 
. But you already know.
However this becomes a known issue in matters of safety when power is cut to the controller unintentionally. As explained above, parts with a certain weight move freely then, which can damage your workpiece and be dangerous to humans (which is one of the many reasons why CNC machines are usually in an enclosure with a safety switch on the doors which stop the machine’s operation and hold the steppers with idle holding current in their position as soon as the door is opened). On power outage, the holding current is absent, which can cause danger,
A remedy for this is to retrofit an electromagnetic brake. It lets the stepper axis move freely when there is power, but when the power fails it clamps the stepper motor axis and prevents it from turning, preventing damage to the workpiece or the equipment or harm to humans. This has recently been discussed here and here, as well as other possible solutions against parts moving freely.
As for the performance of the stock stepper, I don’t think the Onefinity CNC manufacturer would have released the 80 mm mount without thoroughly testing the machine with a 80 mm spindle, there was too much demand for it to deceive buyers with a underdimensioned stepper. They stated repeatedly that they see no reason for a stronger Z stepper.
However if the monster stepper you put on the machine has a magnetic detent so high that it is enough to hold the spindle when power is off, I understand you will leave it there (although in terms of safety, it’s not the right solution. But stepper stock configuration isn’t either)