With proper alignment specs (0 front toe and minimum allowable rear toe), you can run as low as 2" without a camber kit, and you shouldn't run into any issues. I am currently running Eibach Sportlines and don't have a camber kit at all and my tires have been wearing very evenly.
Rarely does a focus need camber plates up front. Based on the data and numerous comments here, front camber plates are possibly a bigger nuisance then they are help. As mentioned just keep front toe setting reasonable (a slight amount of "toe-out" is preferred).
The rear is a different story. Ford specs allow way too much camber. That along with excess toe-in (also allowed by the specs) results in lots of inside edge tire wear. Since rear camber is not adjustable from the factory the cheapest and easiest solution is camber bolts. I advocate that camber be kept to -1.5 degrees or less in the rear and that can only be achieved with the bolts or some other camber adjustment devise whether the car is lowered or stock. Keep rear "toe-in" to the minimum allowable specs. For SVT's and most lowered cars that's 0.04". For un-lowered non-SVT's its in the 0.08" area.