This car needs better dampers and springs far more than bracing or sway bars. The better package will be in good coilovers, front, rears stay with springs and shocks unless you strengthen the mounts for the top of the shocks as they are not designed to carry the load by adding springs around the shocks(coilovers). You can improve the ride, handling and durability over the stock setup with a simple to install solution with little if any weight gain and usually weight loss which is always good in unsprung weight.
Though many report feeling a difference adding bracing, sways, etc, it does not always translate into being faster, keeping the tires planted on the road with better springs and dampers will do far more.
Even before that lighter weight wheels, wider than stock, proper rubber, will gain the most, then the suspension, then I prefer to do bushings and a two point front brace, then look at sways which I am currently on stock with just upgrade front bushings and adjustable end links to dial out any preload.
The Fiesta ST bushings are quite firm, part of why it handles so well stock but rubber bushings tighten when flexed and thus are not quite linear as well as they allow the control arms, etc, to move around under load, not a good thing. Ultimately monoball is the best but not for the street, to much maintenance and noise, urethane with metal inserts they bushings can pivot around are the a good compromise as far less deflection and more linear travel. I did this on a C6 Vette and the ride, feedback and handling improved dramatically even on a car of that level.
Once all this stuff is done then address bracing, better yet a roll cage but not practical or usually safe for the street.
Another option few will or have done, fill the chassis with structural expanding foam, I did a Scion tC and it felt like a fully caged car, before we caged it, it was a long, tedious and messy process and still cost $600 at distributor price but we used 360 liquid oz of foam.
The Fiesta has a very rigid body structure by design and due to crash safely laws, look at how well the chassis has held up in some very hard crashes.