if done correctly with proper piping size and design, it will not have a significant increase in fuel comsumption,
but of course the car will increase in power or torque

a cat back system usually ment for high revving engines such as cars with vaiable timing. turbo cars on the other hand can have them as the do not depend much on the pressures that happen through the whole exhaust system.

for those who dont do high revving and prefer a relaxing drive Below (4000rpm), an ideal setup for me (if u are driving in town) and on most cars will be a 4-2-1 extractor, a mid box just after the flex joint and then a rear muffler. (depending on auto or manual, use an S-type for auto)....  this setup will help improve low end torque that is needed for town driving.

if u like to high rev the engine,there are 3 ways.
1. i suggest u go for a 4-2-1, a midbox slightly in the middle of the car (abt 1-2 feet away for the flex joint) and a rear muffler (again s-type for auto). this will help ur high end and top speed but not make it undrivable in town.

2. the second is using a 4-1 extractor, a midbox placed right at the back just before the  fuel tank, and a rear muffle. i would say that this is best done ONLY ON MANUAL CARS. this setup will work wonders in high end but will be really bad in the low end torque.

3 the third of course is the extractor and a rear muffle only, (but will be really noisy) most of the time....