Loose feather vs tight feather is probably a better description of what we've seen, tbh. I'm sure the Isabel got it from the orpington side.This link from the thread by Miami Leghorn has genetics (tons of genetics) on feather genes -- about 1/3 of the way scrolled down talks about hard/soft feathering.
http://www.edelras.nl/chickengenetics/mutations2.html#gen_mut_feathtype
What you mention too, I wonder what the feathering for heterozygous (one hard feather gene, one soft feather gene) would produce. And -- it kind of seems like it could be possible to have both soft and hard feathered Leghorns -- so all the people who spoke up are right. That feather link is way too complex for what I'm working on now -- It also makes me wonder a bit about slow-feathering and fast feathering. Add to that, someone said there is loose feathering and tight feathering -- so you get geometrical options the more choices you put in the mix.
That's another reason to focus on just one or two traits to 'work on'.