best and cheapest option is to get a later spec mk5 zetec-s fiesta rear beam and also focus 2ltr stub axles and calipers etc.

as ian said not only do you improve the braking but also the stifness of the rear end alot!

i have only had mine over braking to the rear once and that was on the mk6 near birmingham on all of the elevated sections as i was slowing down in the slow lane and the front and back of the car followed different truck wheel tracks..... scary moment!

part from that it has been fine!