The main reason that no one will ever have x-ray vision or even Terahertz vision is because you'd need to be in two places at once. One of you would need to be projecting the radiation and the other of you would need to be standing on the other side to detect the wavelengths that weren't absorbed by whatever it is they were passing through. Superman would need to be not only psychologically schizophrenic but physiologically two people as well. And as the universal speed limit is the speed of light, even if he moved at lightspeed he'd be unable to out-run the radiation he emitted prior to moving position. So he'd effectively be blind even if he could somehow emit x-rays in the first place. Of course if he had a fellow superperson, one could put out while the other received, but perhaps that's more the realm of superporn fantasy than physics.