Those of us that have been foxbody guys for 30 years+ have seen it all when it comes to this topic.
Some notes:
A Saleen rear bumper does not look right on a gt. The side skirts are different.
Cutting holes in a gt rear bumper is a great way to force yourself to buy a cobra valence.
Gt tail pipes look all sorts of wrong on an lx.
Angling straight pipes down to go under the bumper does not look natural.
The cervinis rear bumper doesn't say cobra on it, so it's probably the way to go. Though if you are getting paint work, may want to do the wing too at the same time.
One of the trends back in the 90s was 3in pipes through gt bumpers. I can only imagine how many rear bumpers were ruined by my friends.