While the elimination of passive stress gain is awesome, I feel like this is more than a little unbalanced (in the player's favor). Generally speaking, you're going to build your combat party out of slaves with high Courage (like, 60+) in the first place, and then it's going to steadily increase as they fight. Meanwhile, even cheap armor goes a long way toward reducing damage. I don't know if I've ever seen one of my party take more than 20 damage in a single hit, other than against the golem in the ruins.
You might want to consider making Courage/3 the cutoff rather than Courage/2... hell, maybe even /4
Viewing post in Aric's Expansion Rebred
I may make it with varying difficulties for player preference. However, that sounds like a party built for end-game content. By that point there are a myriad of ways to handle stress effectively and the stress in combat system starts (for most) becoming more of an annoyance than added difficulty value.
This should balance it out a bit and give a reason for Courageous slaves to be built to fight in early game content.