Three questions I have had with not just the fiesta but most my cars when it comes to exhaust, plugs, and filter. It seems like they did all the right things to test out these 3 changes and aren't trying to sell you on parts just trying to improve the car. There is 6 parts to the story that you can go back and read this is just the dyno part.
https://www.fordmuscle.com/tech-stories/the-way-of-the-fist-part-6-dyno-baseline-and-mods-for-power/
https://www.fordmuscle.com/tech-stories/the-way-of-the-fist-part-6-dyno-baseline-and-mods-for-power/