Superhuman AI is obviously possible - humans are possible, but obviously aren't remotely optimal. We e.g. can't at all visualize/intuit 4D space, despite handling 3D space just fine. There are so, so many things that computers already beat the best humans at, even just creating a digital analogue to the human brain design and essentially refactoring it so it's not hardwired around human physical limits (the wattage, in particular) would make it better than every human. Or, figuring out what was so special about Einstein and reproducing it. Or just being able to run a physicist's brain on a GPU, then getting 1 000 000 GPUs and creating the world's largest para-university of physics.