No offense but it seems to me that you should rethink the whole concept
of "the ability of certain westerns
to transcend the usual limitations of the genre and touch on some serious
issues."  It seems to me that film studies research from the past
25 years or so has demonstrated that all Westerns "touch on serious issues."
You might read, for instance, Will Wright's Six Guns andSociety   as
a start.  There is a somewhat recent bibliography in the second
edition of Cawelti's Six Gun Mystique.  Jane Tompkins new
book West of Everything also has a few thoughts in it.