DBZ fans might not like what I'm going to say, but you know what? It takes time for Goku to transform to his strongest level, Super Saiyn 3. It takes time for DBZ fighters to power up in general. Superman wouldn't wait around for this to happen, and he doesn't need time to be at his strongest.
Its true Goku and many powerful Z fighters could destroy a planet with an energy blast. The question is whether Superman would be incapable of stopping such an attack. Also consider that strong energy attacks tend to move slower & Superman could just dodge them.
If you ask me, physical strength is at least a wash. Name me a physical feat Goku's done that Superman couldn't pull off. Goku can smash a mountain, Superman can lift a mountain. To me Goku's only possible advantages are energy attacks and speed because of instant transmission.
Incidentally, I have doubts Goku is capable of knocking out Superman.
I would suggest that it's not the medium, but the quality of perception and expression, that determines the significance of art. But what would a cartoonist know? -Bill Watterson