The smaller turbo'd models had different engine management- Ford's own EEC4 system as opposed to the big turbo ones which used Weber/ pectel etc etc. To use this EEC 4 loads of changes were made, and also changes were made to the whole car, to make it a nicer and better performing car in standard form.
The injectors are bigger, but cannot be changed for greens, blues, grey etc etc. As were some of the engines sensors.
The furl rail and fuel regulator is different.
The inlet manfold is different.
The throttle is larger.
The intercooler was changed.
No distrbuter, just coil packs.
Different map sensor.
No coil or ignition amplifier.
Head was modified as was cam cover & belt cover.
Oil pressur pick up system was completely changed to allw better spray pattern onto bottom of piston crowns.
Exhaust mnifold- turbo was changed.
BHP peaked at 227@5750rpm as opposed to @6250 on larger turbo models.
Externally fuel flap and mirrors were from esc mk5/6 face lift.
Badges on back were changed, and the suspension was also altered.
It also became an option to have the car with no whaletail.
I think it was made a little more user friendly due to Ford not needing to meet certain criteria to enter the WRC homologation crap, hence no charge cooler or water injection etc as with the larger turbo models.