Goku doesn't use magic.
But he does use some sort of mystical power. So it's an open question, then, as to whether Superman is vulnerable to them in the same way that he is vulnerable to magic. I would expect that he is.
I've never actually watched DBZ, btw, I was just going by the Wiki article.
Given the massive power level inflation that happened in DBZ, I think we would reasonably have to consider where in the series we're talking about.
Fair. Though to be fair, we need to apply that to Superman as well: Which incarnation of his character are we talking about, here? In some cases he can be thrashed by a big robot, in others he can be chucked at the moon so hard it cracks in half without him suffering injury.
Also, Superman's powers are seldom really tested. He's so much more powerful than most everyone else in his universe (Darkseid is a notable exception, but there just aren't that many others) that he has no way to really train. Give him the opportunity to train like Goku did, and to fight opponents who can actually give him a challenge, and it's a whole new ballgame. Even if his powers are charged by sunlight, there's still skill in how to use them.
True. When it comes to Superman actually fighting, he's not all that skillful - which really comes up against folks like Darkseid, or Mongul, or Doomsday. Or, presumably, Goku. This ties in with your other point about different incarnations and points along storylines - some have practiced more than others.
