According to spec and all reports from users, it is 6fps regardless of power source, file type or bit depth. For those wanting 14 bit NEF images, it is much faster than a D300 but lower than the D300's 12 bit or JPG mode by 1-2 fps depending I the grip is used with higher voltage batteries. If you are shooting subjects were 1 fps really makes or breaks your work, you should be looking as the D3s.
I share Stan´s ideas. I have also a feeling that Nikon itself has announced that the grip does not increase the speed. That is my experience, too, after two weeks use of the grip and one months practice with my Nikon D7K.