He's actually solar-powered, with everything from his reflexes and strength to his cellular regeneration enhanced as long as he's been in sunlight recently. But, yes, it is physical. Superman can't become significantly stronger just by breathing a particular way, focusing especially hard, or having a particular mindset like DBZ characters can. Eventually, in a story set in the future, Superman had permanently absorbed enough solar radiation that even krpytonite barely affected him any more and he survived a nuclear blast. In any case, Superman is still vulnerable to magical attacks, so Goku certainly has a chance if we consider chi manipulation to be magic, which we should.