I might be able to offer some better advice than some of that offered already mate as I've had my focus mapped by James and Kenny about a couple of months ago. Had a custom map done by them to suit my spec as follows:

-AEM style long arm induction
-ST170 4-2-1
-Power Engineering Decat
-Piper Full Cat Back
-Stage 2 (281) Comp Cams
-Kent Cam Verniers (4deg in adv, 2deg exh ret)
-CFM UDPs

Had a custom map done by James and Kenny to suit my spec. Haven't had it on a rolling road though to see the exact gains but all I will say is as soon as I drove it out of the little industrial estate that MSD is in, the throttle response was immediately different. James mentioned rich top end so the fuelling up there was trimmed and also apparently my ignition map was altered quite a bit but not so much fuelling. James would be good to speak to as he should be able to remember what he did on my car and any changes he and Kenny made. On a different note I went to Santa Pod 2 weeks ago and cut my 1/4 time from 16.5 to 16.1 and pulling 5mph more on terminal speed from 83 to 88mph. Top end definitely a difference. Anything from 4k rpm upwards and its party time! I drove all the way down from LONDON to have my car mapped by them so put it that way.

Forgot to mention my car is a 2002 1.8 Zetec as well. Cam spec:

281 Duration
0.381" lift
Duration at 0.050" lift: 228

Both inlet and exhaust cams are same spec usually exhaust cam is slightly milder but not in this case.