Agreed with Lance, 5' in one growing season is pretty amazing for any tree. I can't think of a tree off hand that grows faster than hybrid poplar, but they would be lucky to do 5' in one season after being established.
Usually when you see a nursery or mail order catalog state "up to
of growth per year!" you can expect half that after the first growing season. To meet thier claims the soil would have to be ideal for the species, moisture level just right, perfect climate, and the right type and amount of fertilizer used. And the stars would have to align properly too 
Anyways green giants should make an attractive & troublefree hedge, just not as fast as advertised.
