Replacing the turbo is one of the last things people do it and it truly changes your car into an entirely different beast. Typically with the Fiesta ST people look at installing larger larger turbos so that they can reach numbers of 350+ whp.
In a straight line these turbos will make a significant difference in performance and sound. Around corners these larger turbos can be an issue as it becomes easier to drop out of the boost rev range. If cornering at maximum g-force and the turbo kicks in this can set the car off balance and send the car into a wall.