Moreover, none of this has much impact on the fact that measuring the power of an optimiser simply in terms of the relative size of a target subspace to the search space doesn't work: sometimes tiny targets in massive spaces are trivial to solve, and sometimes bigger targets in moderate spaces are practically impossible.

I thought I described my attitude toward this above: The concept of a problem's difficulty, the amount of mental effort it feels like you need to exert to solve it, should not be confused with the optimization power exerted in solving it, which should not be confused with the intelligence used in solving it. What is "trivial to solve" depends on how intelligent you start out; "how much you can accomplish" depends on the resources you start with; and whether I bother to describe something as an "optimization process" at all will depend on whether it achieves the "same goal" across multiple occasions, conditions, and starting points.