Can you even get butt sag in a coupe?
I can't tell by this photo if you have what we call "butt sag." The way to know is to measure the door-to-quarter panel gap. If the gap is appreciably wider at the top, you might have the butt sag.
If you don't have a widening door gap then what you're describing is a simple low suspension. Most of these cars have an adjuster on the spring plates in the back. You should be able to make the back end set a little bit higher just by turning an allen bolt on that adjuster.
If you don't have these adjusters then you'll be looking at reindexing the torsion bars, which is a PITA. Very unlikely that's needed though.