This is kind of similar to another issue I had with the mirror mod, but this time my model that has the mod shows the mirrored version going through the main one. Even if I reposition the pivot point or 3D cursor so it's in the centre, it makes no difference, it never appears in the right place - opposite the main one.
It looks like my shoe model has far too many vertices and has made the file over 38Mb. Not sure of the best way to fix that. I've tried the Decimate modifier, but that ruins the model. :-\