I read an article on modern digestive issues with wheat, which although perhaps simplistic in some of its conclusions, does raise some interesting points:
There's little question that sourdough does something that affect the glycemic index of bread, though I'm not sure if they've identified what the agent is or what it does, but I think that would affect the starch, not the gluten proteins. There are a number of journal articles on sourdough, but I've not tried to read more than a couple of them. I'm not sure if anyone at UNL, where my wife works, is doing research on breads at the moment.