Captain, you read my mind (question). I have been researching the same question. It seems that both the americana and biwaenesis have the twisted leaves and they differ in their maximum size and how tight the curls are, as far as I can tell from the book I have (by Peter Hiscock). The pictures show the americana (max height 20 cm) to be less tightly twisted than the biwaensis (max height 35 cm).
However, a confusion for me is that the book also indicates that "V. asiatica var. biwaensis" is also known as "V. spiralis" and "V. tortifolia". Then on the very next page, it lists "V. spiralis" (same name) as a separate plant but with a common name "straight vallisneria", which of course is straight and has no twists in the leaves, and it grows to a maximum height of 60 cm.
Hope this helps.