You have to be cautious when selecting a power pulley. Depending on the car, replacing the crank pulley with a smaller one may underdrive several other accessories such as the water pump, air conditioner, alternator, power steering pump etc. Each of these is designed to operate within a certain RPM range. That's why each one has a specific size pulley on it. A properly designed underdrive pulley kit usually includes pulleys for the accessories as well. This assures they maintain the correct RPM/operating range.
Example- an alternator needs to charge the battery when a car is idling. Most alternators must be spun at about 1500 RPM (before they begin to provide a charge) and a maximum RPM of around 3500 RPM to acheive maximum output (this differs by manufacturer, naturally) If your cars idles at 750 RPM the alternator will have a pulley about half the size of the crank pulley so the alternator will spin at 1500 RPM and provide some juice. If you slapped a smaller crank pulley on and not change the size of the alternator pulley you would reduce the alternator RPM's and output.
All benefits of underdrive pulleys occur at higher engine RPMs because you don't need to spin the alternator as high to acheive maximum output (the same is true with other accessories). Using my example above. If I'm crusing on the highway at 4000 engine RPM my alternator would be spining at 6000 RPM. And, as I mentioned, peak alternator output occurred at 3500 RPM. By reducing the the alternator RPM in that 3500 to 6000 range by adding an underdrive crank pulley a small amount of HP is freed up. The problem is that the alternator will not be as efficent at lower RPM's.
The conclusion is that underdrive pulleys make sense for high speed continuous operation but can cause problems at low speed operation such as reducing electrical output, overheating (spinning the water pump too slow) and sluggish steering (spinning the power steering pump to slow). For normal driving they probably create more problems than the benefits from the small power amonts that result. If you are looking for every last horsepower, there are some small gains that can be found by doing this....but understand the potential downsides for normal street driving.
the afx is one pulley. the only pulley that needs to be replaced on the focus is the crank. its not so much the weight as it is the size of the pulley that gives u power. smaller the pulley the more underdriven your accessories are.
the only reason the FC kit has more than one pulley is to keep the same belt size. the water pump pulley from FC is actually larger than stock and i believe played a roll in the one time i over heated when it was 100 degrees out and i had my a/c on.