This study describes a part of the research, development and testing effort that led to the typical German shape of the more recent U.S. standard helmets (since mid-80's).
It's amusing to see the effort involved (and the duration, apparently - the PASGT helmet wasn't introduced until almost a decade later).
In the end, they arrived at a well-known shape that was basically a simple gut feeling product, created during WWI.
Sometimes a lot of effort and deliberation simply doesn't yield a superior product. Access to decades of advances in material and production technology often does the trick more easily.