Originally Posted by MyThIc3LiTe
Say goodbye to your warranty for starters.
It's a fwd car...rear discs are a pointless luxury since the front really is doing all the work.
Not entirely true. Sure on a FWD car, the front brakes do most of the work, but then again they do on a RWD car also, but not as big a percentage. However, since disc brakes are more efficient than drum brakes, the benefits of having rear disc brakes on a FWD car is that it reduces "front end dive" under fast breaking conditions. The 2011 Fiesta had good stopping power, but tended to dive a bit when braking hard. My Focus has virtually none of that in comparison.
Perhaps if a Ford dealer did the conversion, it would still be under warranty? If you can't do the conversion, then I would at least get some alloy (spoked) wheels. Would help to shed a little weight and perhaps provide better brake cooling.