Lots of *load* (millions of devices) hanging on power lines, each one an eventual path to ground.
I would think that the spike induced in an unloaded power line compared to a loaded power line would be quite different in amplitude.
“No way of telling what a CME induced power spike would, in reality, do.”
It’s not really a “spike” - it may be 30V/km in induced voltage. It is “quasi-dc” and it will build slowly.
The danger is these DC currents running through transformers - causing half-cycle saturation.
An unloaded grid would be impacted very little. Better yet, shut down the grid.