Cervini's LX rear

I'm not sure what LX bumper you mean. Do you mean Cervini's Saleen style rear bumper? If so, the problem would be that it's not a complete bumper. It's a bumper extension that mounts to the bottom of an LX bumper. So for you to use it, you would first have to install a regular LX bumper, then mount the extension to the bottom of it. Also, the moldings (the part of the trim that lines up with the door moldings) are different on GT's and LX's, so the rear bumper's molding wouldn't match the rest of the car's.
Even though this bumper setup would be extended downward with the bumper extension, it is still higher than a GT bumper. So the rear of the car would be higher than the sides and would look disproportioned. The best option for a GT is the Cobra bumper. The moldings match the GT's, and it's designed to match up to the lines of the GT trim... so it looks like it came on the car, not added later.
Hope this helps!

If you are replacing the rear bumper with the cobra rear, you will also need to buy an lx/cobra exhuast, or you could just run with no tailpipes (that really loud though) most of the time the exhuast also has to be modified to fit with the cobra rear because the hangers are not long enough. just food for thought