Coilovers would still just be a shock and spring in the rear so they mainly improve the fronts. So, setting aside for now that the fronts would be true coilovers, the rear torsion beam is just a simple beam that goes across to both wheels suspension and can at least "flex" somewhat independently but nothing like having multiple points of a true IRS multilink setup, which actually does have a few cons of its own too, like getting camber just right and cost/complexity for limited improvements over torsion beam. You might be surprised to hear even the Civic Type R is torsion beam, remember unlike a solid-axle in a truck, they have advanced to where there is some independent flexibility and handling doesn't suffer much, but ride quality over rougher roads you will feel the whole rear shudder sometimes.
Anyway, there are plenty of lowering springs out there and some Japanese companies like Auto Exe have lots of bracing, suspension bits and a front sway bar etc, but are not exactly very inexpensive. I have found that meatier tires help with the cheap feeling rear torsion shudder as well by simply helping to absorb the vibrations. But if you want to do lowering springs or coilovers, yes, they still help tremendously, the rear torsion beam just doesn't have quite of a refined ride of full IRS.
