One of the ways Superman's sudden increase in strength is explained away, preventing viewers from thinking it is corny: http://tvtropes.org/pmwiki/pmwiki.php/Main/WorldOfCardboardSpeech
I'm not sure we'd accept this. I do not even remotely understand the logic being applied here. I also don't see how it's any more logical than flying past a portal so fast that you close it.
Could not have said it better. Decide how you want your world to be, system etc. Create limits to magic so that your MC can have enough of a hurdle to overcome. (If he or she is the magician that is.) It's your palate, play with it but there should be a bane of sorts to what can be done. Opposition creates interest but make it your own. Good luck.