When you refer to the elastic grip do you mean that you move the spear (or javelin) at a flatter angle and keep the fingers looser? If so, it would explain one of the other parts in 'Storm of Spears' where it mentioned that the overhand grip was less effective (for Australians) because the wrist doesn't have a great deal of vertical control in that position. So a looser grip on the spear may make the attack far more accurate and powerful. However, this has another set of problems in that it may weaken the grip on the spear and result on it being dropped, probably pretty bad in a fight.

It is interesting when you say that Australian sports that include throwing (cricket, for example) do not use the overhand throw and yet Americans do in their respective sports. The overarm throw is very common in Australia (I should know I live there ) so it seems strange that would effect his testing in such a way. It is possible that someone with practice could be far more efficient with the overhand thrust than someone with little experience in an underhand attack. But a underhand attack may be easier to learn and use efficiently, even if an overhand attack with practice may be better (or not, given highest force was the same for both attacks).

However an overhand attack has slightly less range than an underarm attack, something of a great deal of significance when fighting with spears (Macedonian Phalanx>Greek Phalanx). However this is something I am not completely sure of so feel free to correct me.