I'm actually thinking that he took extra mild physical consequences. We see multiple occurances where he just keeps going after taking a beating.
There are multiple times where lines like the one in Small Favor come up: "He hammered punches into my short ribs, and he knew how to make them hurt. Unfortunately for him, I knew how to be hurt."
So, I'd say he likely took extra physical consequences. Since you can take backlash as physical consequences, he could use those extra mild consequences to either take more of a beating, which he usually did, or to use more powerful spells even with his lower discipline...