It seems an awfully specific feature. I don't think many people would have a use for it, and I am loath to add clutter to the interface with such features. That way lies feature creep.
Yeah, I know, I really need to optimize this. However, there is a reason I use FSRefs and not paths in the list, which is to keep track of files as they are moved around. However, calculating paths once before sorting and then caching them might indeed help. I'll look into that.