You can't "squeeze more power" out of the fiesta or the focus, unless you get the focus ST.
You can't do an ECU tune yourself. Not possible. An exhaust will get you nothing.
On a base focus or fiesta (2.0l), an intake/header/exhaust will cost you $1500 and get you MAYBE 5-8hp. If you get an ecoboost 1.0L turbo (much more expensive than the base model), that same $1500 will get you 25-30hp.
Both are good cars. I'm seriously considering trading my G35(parting back to stock and selling all go fast parts) for a Focus ST, and going big turbo. The stock block is good for 400hp or so, which is alot in a FWD hatchback.