If you dont want to put your car on the dyno then adjustable cams wont be your thing. People have picked up as much as 5-10 WHP. the other most "easiest" mod that gives you more is the Full SVT exhaust w/ Tune. I think I was able to get around 5-6 WHP more in my tuning when I was on a dyno adjusting. It was fun, got some good dyno plots (did some 25+ runs) and was able to work with Tom on my Tune in TeamViewer mode and get adjustments on my Tune also as plots were sent in real time.
You said it, this is not a V8, so any changes to up the power with out going Forced Induction are going to be costly and take machining and some of it custom work.
There have been only a few that have gotten close to 200 WHP on a Zetec motor naturally aspirated. and they spent a lot of time and money on it.
get a good engine, 3k for a used kit, $500 in tuning you will have a 250 WHP turbocharged Focus.
Just a note on the coil and wires. Ford has done an amazing job on the ignition system on the Zetec Focus. Most who have large amounts of power can still use the stock coil and not have to worry about blow out of the spark. On a Zetec Motor in a Ford Focus replace the coil with OEM only when the old coil brakes and replace wires with what ever the part store carries that has lifetime replacement and just replace then for free every 3-4 years or as needed.