The Fiesta ST has the potential to gain some significant power without modifying the car in any way other than an ecu flash. These tunes can produce up to 20% wtq depending on the level of aggressiveness and reliability you desire from your tune. These tunes continue to be extremely efficient as you modify the car, allowing power gains from modifications that may only increase cooling.
One of the best companies in this business currently is Stratified. Stratified offers tunes through the mail either attached to an access port or as standalone for you to install onto your own access port. Their tunes offer different driving modes as well as specific tunes for your modifications. Starting at only 50$ these tunes can produce significant power and sound notes for the price.