Barry, are you sure that the 170 as the 200A is a half stop faster than the 200 since the glass of the 170 is smaller (72 mm vs 82 mm of the 200A) ???
They're faster because the cameras are more sensitive to light not because the lenses open up to a wider aperture. The HVX200 had an ISO rating of about 320, Barry has reported the hvx200a and hpx170 to have iso ratings of about 500 which is around a half stop faster.
davide