You did not state what type of fiber you are using and its diameter. It all depends on the aspect ratio (length decided by the diameter) and the interfacial adhesion. Typical fiber has a diameter of 10-20 micrometers. Thus, assuming this is the case for your fiber, your fiber's aspect ration is around 600. The fibers with aspect ratio of 1000 act almost the same as the continuous fibers. Thus, the fiber with this length will give you very good reinforcing effect, assuming that the fiber surface property is compatible with the concrete material and have good adhesion.

Yes. It adds strength to the concrete mixture.