For various historical reasons all non-composite fields are cleaned when used in a save path. That won't change.
You can work around this by applying a function to the component when you use it in a template. The function is applied after the component is cleaned. Example:
Code:
{#kobopath:re(_,/)}/{author_sort:sublist(0,1,&)}/{title} - {authors}
which will convert all underscores in #kobopath to slashes. If you don't want *all* underscores converted then use some unique string to mean slash, for example ZZZ, then use the template
Code:
{#kobopath:re(ZZZ,/)}/{author_sort:sublist(0,1,&)}/{title} - {authors}