Today I picked up some used books on a lark. One was The Wars of the Ancient Greeks by Victor Davis Hanson. I figured there wouldn't be anything in there that I hadn't read before, but it was only a fraction of the list price so I figured I might as well. To my delightful surprise, in the back of the book are statistics on weapons: weight, impact area, etc. This is pretty nifty, but it certainly adds some evidence to the argument on spears wielded overhand and underhand.

For the same spear wielded by the same person, overhand is not only faster than underhand (55 ft per s vs 24), but also more powerful (70.8 foot-pounds of force compared to 13.5).

Those are some impressive differences in performance that speak for themselves.