Personally I hate the yellow/orange Cage display, I always turn that off (Subdivision Surface rollout, Show Cage checkbox). If you're using Symmetry set to Slice & Weld, the overlap doesn't matter. And when Show Cage is off, it's hidden too.
But if you need to fix it... go into Border sub-object, select the seam border, then in the Edit Geometry rollout hit the X button next to Make Planar. Flattens it. To move to X=0, right click on the Move tool in the top toolbar, make sure the transform center is on Use Selection Center (the icon with the two gray bars and the red dot between them), and type 0 in the X field.