This is mostly a quick question. My understanding is, the real frequency graph that you will perceive is the result of the interaction between the headphone FR (assuming some standard testing equipment) + the geometry of the ear canal?
So state of the art headphone FR = standardized tests on any headphone model + do an MRI of your ear?

Are we at that stage to get such EQ? And the only tradeoff you would get is some delay due to EQ processing?