The Stamile cultivars may provide a possibility for isolating the foliage type link to hardiness. Take a random sampling of northern hybridized Stamile plants and southern ones, and cross them. This may create some hybridization environment-neutral plants to test. Take the seedlings and grow them in both northern and southern environments and compare the hardiness of dormants versus evergreens in both environments. Of course still many variables cloud this not the least of which is random variations in the seedlings, but it still may provide some insight.
The scientific endeavor to establish any causal relationship between foliage type and hardiness is worthwhile and interesting. However, from the practical viewpoint of people growing daylilies in their gardens, the simple question is which are more cold hardy as a matter of common availability ? Snow cover is a huge factor as plants covered with serveral feet of snow all winter long are greatly insulated compared to plants in exposed ground. However, I'm still inclined to believe, for reasons mentioned in my previous post, that dormants are still generally more likely to be cold hardy.
