What I said most importantly :
Transonic drag being far higher than low subsonic drag, low rpm is better either for your eng (max pow dive) and for your total drag coef.
But I hve to agree that was not my best writing
I think that your answer lies in faisability (large series) and techniques.
Just remember that the very goal of the Clark's Y airfoils series is for an easy craftsmanship (russians abused of this with their wooden series of La, Yack etc... all were made out of Clark Y).
Propeller material:
German = wood (but some 109 had metal..... seems it was not so much a prob ?)
US = casted aluminium
It makes a huge difference in what kind of airfoil you can achieve.
But still it's only my own guess.
Thx for making that thread an interesting one. Pls go further on
EDIT :OOOpss..just forgot to say that the Clark Y airfoils had a flat bottom to ease marksmanship.