correct on adding a sway bar to a stock suspension ST vs a modified suspension ST. As you change springs, tires, dampers, sway bars, and bushings, the balance of the car changes. Either way, changing the driver technique can greatly alter the balance of the car. There are some really great videos online developed by Ford of Europe where they interview the different engineers, including the one who designed the suspension. The video shows the Fiesta drifting nicely around corners. I have found that I can do the same (not as aggressive as shown on the video) by applying left foot braking and balancing the car with throttle and brake, which is exactly how I was taught at Team O'Neil. Even the stock Fiesta, non-ST, can become very tail happy with this technique.
One concern that I have with modifying the suspension on a car with ESC is that the system is designed to work with a certain suspension setup. As you further modify the suspension, you move outside the parameters set by the ESC engineers. At that point, you are better to run the system completely off all the time.
As for running "coils", all Fiesta's run coil springs. Actually, they also all come with coil-over-strut front suspensions from the factory. When I hear people say "I am on coils", that can mean anything from stock to a fully adjustable push-rod suspension. They all run coil springs as opposed to leaf springs, torsion springs, or air springs. I guess that is the suspension engineer in me coming back out.