FI is the way to go for the most and easiest power increase-thats not to say that you can't give a NA car a real boost in power but it takes a fair amount of work to get there.
Tom is right-turbos are best for ultimate power but from what I have gathered on the Jet they require skill, hard work, and maybe some luck to get fuel management right and keep the engine from blowing up. My experience is with the JRSC which is truly a bolt on upgrade that is well engineered, reliable, has no fuel issues and very importantly, is CARB compliant. It's also expensive and doesn't give a huge power increase but it does respond to further modifications to increse its modest power boost(I can't wait for the boost upgrade to come out!). It's made specifically for the Focus and works very well with the Zetec engine. One thing about NA is that you can go that route a small piece at a time whereas with FI it's one big cost up front.
2000 ZX3 W/ JRSC, JBA, FC 65MM TB, Cool Flo MAFS, Random Tech Cat, MBRP cat back, Modified Volant, standard Superchip.