Thanks RBF, I did my modest math, and got very similar results. I figured I must have made an error along the way, and set my results aside. Like a few other planes I've dived, the Caravan will build up speed very quickly when pointed down (as opposed to a few floatplanes I've dived, when once pointed down, I had to add power to achieve the required speed).
The two photos in the preliminary NTBS report do show the airplane descending vertically, probably already broken up, and they can probably validate the speed with those photos.