The weapon bonus from 'magical' strength is written into the power. For example, Mythical Strength is +6 to damage. Mechanically, it doesn't matter whether you're picking up a telephone pole and swinging it or using a metal baseball bat...both should end up at Weapon:8. There is a significant difference to the story, one is a ponderous swing of a heavy object and the other is a fast smashing blow. But mechanically, both are clubs with a +6.
Remember, FATE in general and DFRPG in particular don't simulate real life - they build narratives / stories.