Have you adjusted the "nut behind the wheel" and added grille blocks? Lowering the car is going to have a relatively small impact on MPG.
If you go that route though there are cheap springs available because of the Focus' popularity. For $76 shipped you can get a 2" drop (that will look great) semi-properly without taking an angle grinder to your suspension.
DNA SUSPENSION LOWERING SPRINGS RED 00-04 FORD FOCUS | eBay