There are several different nandinas, as you probably know, you would want the tall ones for a privacy hedge. In my experience, they are fairly slow growing. I think pyracantha grows faster and gets taller, as I've seen it grown like a climber on a wall or trellis. Also beware of the thorns on the pyracantha that I understand really hurt ("firethorn").

I wouldn't do it that way. Nandina is slow to establish, but usually takes over after a while. Pyracantha makes a great cover for fences and an impenetrable barrier too. If you want really fast and thorny go with Cherokee Rose. It needs no care either.