Many people mistake Superman being vulnerable to Magic as being the same as Kryptonite. Of course it isn't. Magic just negates his invulnerability, if he gets around a magicians spell he can still knock them across the park. Kryptonite (green at least ;P) doesn't just zap his powers, but slowly kills him. Just a distinction so many overlook. Why I'm looking forward to the rematch of He-Man vs Superman in the injustice comics. I once read an article years ago that talked about how technically...Superman is faster than the Flash. I don't know how the science holds up but basically it boiled down to this. An object weighing as much as the Flash weighs (about 195 for Barry Allen so we'll use that) would cause a lot of havoc on the environment around him moving at the speed he runs. But the Speed Force acts as a buffer for that allowing him to move through the atmosphere with minimal affect. Superman however does not have the buffer of the speed force and also weighs even more ( 235lbs is generally given). If he truly moved at his top speed on earth he would cause massive damage (like Neo in the Matrix when he was rushing to save Trinity and was upturning cars and breaking windows), but in space he can move at his top speeds. Which haven't been measured.
In truth, Superman is as powerful as the comic writers need him to be, and he keeps getting more powerful. There was apparently a comic where he "heard" the signal from Jimmy Olson's signal watch several light years away (which is impossible, but that's not the point) and arrived in minutes to help Jimmy. That's insanely fast, and likely faster than Flash can go. Though, it also depends on which Flash. Barry Allen generates the Speed Force that the others tap into, making him much more powerful and faster than the rest of the Flash family. Though, Wally West was responsible for "rebooting" the whole DC Universe at least once.
Superman is also so powerful that he can apparently hit something so hard that the blow will shatter whatever planet he's on at the time. That's one reason he holds back in fights, even against even other super powered beings. His own powers literally frighten him.
No, Magic doesn't kill him, it's just much harder for him to defend against or combat. It can also be used to incapacitate him. But, he has certainly taken down many beings with magic powers.
I agree with both of you. Superman holds back his true strength since he's so strong as you two have mentioned, that he can destroy entire worlds with just a punch. I think the only time that he got close to using his full strength was when the Joker tricked him into killing Lois in the Injustice comics.
