Welding a cage to plate is pretty hack IMO. Also not nearly as strong or safe as being welded to the frame under the car, or to a sub structure like moly tubing under/in the floor.
I think the outrigger welded to that sill plate would need a plate, but not totally sure, because the opposit end is welded to square stock/frame. Unless it passes through to another part of the chassis/tubing.