Depictions of Underarm Phalanxes

[PMBardunias]

although oddly enough by far the most penetration came out of outright throwing the spear (maybe it would have been even greater with a throwing string?)

This is to be expected and for the same reason that overarm strikes are more powerful than underarm strikes. It has to do with the range of motion of the arm and the increased amount of time it can take to accelerate the shaft. Also, you engage more, and more powerful, muscle groups. Someplace on here I did a post on how the amentum functions mechanically. It does not follow the same mechanics as a sling, by increasing the radius of the arm, but rather acts to keep the arm and spear in contact for a longer period of time. So yes, it would be more powerful.

Although I don't think it takes much strength to penetrate human skin while the underarm grip still maintains the reach advantage and, if using one of the braced positions suggested by some reenactors, offers significantly more lateral control.

You have to think of strikes as laying on a bell curve, so the percentage of strikes that land strong enough to pierce anything will increase drastically with the overhand strike. In a line of at least two ranks of dorys, I think you can sacrifice the type of sweeping lateral control you get with underhand for a "pecking" fore and aft strike where the vector can be shifted laterally for each forward stroke to hit targets to both right and left over a larger range than underhand can give.

Reach advantage is relative, and clearly not the most important driving force or hoplites would have all used sarissa a few hundred years earlier. In fact, I think the need to strike overhand in the hoplite phalanx is one reason they did not do so and spears topped out at about 8' with extensive tinkering around with tapering and weighting to give maximal reach (about 5 1/2'). The pressure keeping them holding it overhand was probably the diameter of the aspis when coupled with close formation both along ranks, which made bringing the spear up between touching shields impossible, and along files which denied the room to clear some 5' of spear from under the shield without killing a man behind (or rendering him having progeny unlikely ).