Underdrive pulleys were proven to provide an increase in horsepower and torque years ago on the old 5.0 Mustang. Now it wasn't much, but it did help improve efficiency. The key is not to make them too underdriven and then affect charging and cooling.
I do not believe they will add as much hp and tq as they would on an old 5.0 given the size and output of the 2.0 DI Ti-VCT engine, but there would be a couple and the engine may feel more "rev-able."
I would buy a properly engineered set.
The SR Motorsports set posted above is of good quality. I have used them on Mazdas in the past, but didn't realize they made them for the Focus.
I believe I had a Focus Central lower pulley on my old ZX3 years ago (I believe that was the brand). Not really a noticeable power increase, but it did seem to rev better. Might have been in the my head, but it did feel that way.
On a FS-DE Mazda 2.0 (Protege 5 engine), I noticed nothing. Of course that poor engine was as unresponsive to modifications as you could get.