The past couple of years has seen various groups claim quantum advantage/utility only to have their experiments efficiently simulated with classical methods, notably using tensor networks.
My question is then, is it, in principle possible to know/measure the classical difficulty of simulating a state $|\Psi \rangle$ prepared within a quantum computer without incurring exponential costs?
Some connections to classical difficulty can be made as bounds on the required number of $T$ gates is determined if one measures the stabilizer 2-Renyi entropy $M_2$ (see equation 12 in this paper). You could then quantify the cost of a Clifford + T simulation of the circuit, but, measuring the $M_2$ faithfully requires an exponential amount of measurements.
Another paper introduces the Bell Magic $B$, formally related to $M_2$, but may be measured with low cost. Unfortunately, the same bounds on the number of $T$ gates (or other connections to the difficulty of classical simulation) do not seem to exist (or at least, nobody has determined them).