I agree with your overall thesis here - but am not sure just how broad a definition of fiction you are considering. Do gedankenexperiments count? The physical situation in those has not actually been constructed (so, in that sense, they are fiction), but they are still helpful in illuminating the consequences of different physical theories. As you wrote, they are useful because of our computational constraints, prompting considerations of consequences of our current theories in regimes that we would otherwise be unlikely to explore.