The reason is the S/SE/SES platforms have no support for their ECUs meaning there is no way to adjust fuel and timing to take full advantage of the power adder.
You could add a turbo or supercharger, but 4-5psi would be the max you could use before you run into fuel and timing issues.
2014 Ford Fiesta SE Hatchback
Cold Air Intake
OBX Header with high-flo cat, resonator, and DC Sports muffler and 2.5” exhaust
N2MB Two Step
17” Focal with 205/45s
3M Dark Smoke Vinyl Wrapped Lights
Dean72 gave me solid advice and told me to add nitrous. The reason nitrous will work in these cars is that because it is an oxidizer but does not add pressure to the manifold while still allowing the engine to burn more fuel. The MAP sensor in these engines will get angry with boost and force the throttle body to close down to compensate for the unexpected boost. You can "trick" the engine into making more hp with nitrous because it helps burn more fuel faster without making the MAP angry. You can view my build here . I'm almost done with the install and will be going to a dyno to post the results. If you decide to go with nitrous and have any questions, let me know, I might be able to help.