I tested the change with the authoravatars shortcode as follows:
[authoravatars avatar_size=78 order=bbpress_post_count,dsc page_size=40 user_link=bbpress_memberpage show_name=false show_BBPRESS_post_count=true]
I believe the sort is working but I can't tell for certain because I am unable to display the BBPRESS_post_count with each avatar. As you can see from the code above, i have included show_BBPRESS_post_count=true but no post counts are appearing when I mouseover the avatar.
As you can see from the code, I am also using paging and have a page size set to 40 avatars. When I page forwards to page 2 and back again to page 1, the avatars on page 1 are all replaced with the grey man.