Here's a few authors you might like:
Morpheus -
http://www.fictionmania.tv/ (Do an author search for Morpheus.)
Nomdreserv -
http://www.thechangingmirror.com/php...forum.php?f=16
Tainted Sins -
http://taintedsins.yuku.com/
As you go down that list, they get progressively more cruel. For Morpheus, check out his "Role Exchanger" stories first. Nomdreserv and Tainted Sins mix in a lot of other TFs as well, so I'm not sure if this is exactly what you're looking for. It seems that most people don't know about Nom or Tainted since they both stopped writing a while back, but I still think of their work as being classic, hardcore TF. Hope you find something you like in their work.