glassarcher welcome to the Seven Forums.
But correct me if i'm wrong, shouldnt the GT 520 be way better than an old AMD 4850?
Sorry to say but you went from what was a medium-high end card (a little more high than medium), to a low end entry level card. Not even on a good day is the GT 520 a better card than the HD 4850, that is why your WEI score went down and also why you are having issues running games at high settings. You went from a card that depending on the game can run it at high settings without breaking a sweat, to one that would struggle with the same game on medium settings (and quite possibly low settings).
Only solution is if you still have it is to go back to your HD 4850 and return the GT 520, or look for something higher up the video card food chain.